Shaping up to be one between Piper, Jason, Reyna, Percy, and Annabeth from The Heroes of Olympus, in that order.
Never fully comes to fruition, but there are hints at Reyna liking Jason and possibly Percy.
Edward Cullen from Twilight loves Bella. Bella loves Edward and Jacob, but she loves Edward more. Jacob loved Bella but then falls for her unborn daughter. Mike, Eric, and Tyler all have deep crushes on Bella, but she ignored them. Mike dates Jessica but breaks up with her. Tyler dates Lauren. Rosalie was supposed to be Edward's mate, but she ended with Emmet. Alice is with Jasper, but she kind of gives off lesbian vibes for Bella. In the end, everyone of note loves Bella in some way.
The Host also fits this trope. Melanie loves Jared so much that the alien parasite controlling her body also falls in love with him. However, it's implied the love is mostly physical. Ian has a crush on Wanderer, and Wanderer finds herself falling in love with him for his personality. Even when Wanderer is given a seperate body from Melanie's at the end, Melanie has a slight crush on Ian but loves Jared and Wanderer still has a slight crush on Jared but loves Ian. And this isn't even including the romances between the side characters.
At the heart of Fyodor Dostoevsky's The Brothers Karamazov is an intricate Love Dodecahedron which isn't really fleshed out until the later chapters. It starts with Fyodor Karamazov and his son Dmitri (the eldest brother) who are both smitten with Grushenka, the local Hello, Nurse!. But in love with Dmitri is Katerina, an aristocrat who is smitten with him for some services he did for her in the past. In love with Katerina is Ivan, the middle Karamazov brother, but he maintains that he doesn't really love her. Grushenka herself plays with Fyodor's and Dmitri's affections, but really she's been pining after Mussyalovich, a Polish gentleman who spurned her five years ago. Grushenka also shows some attraction for Alyosha, the youngest Karamazov brother, but his attentions are turned towards Liza, a young girl crippled by a fever at the outset of the novel. But towards the end Liza shows more attraction to Ivan than anyone else.
Also by Dostoevsky, The Idiot. Myshkin loves Aglaya, and is briefly in love with Nastasya but mainly just pities her (which nevertheless makes Aglaya see her as a romantic rival). Gavrila loves Aglaya, but for various reasons won't commit himself to her; he doesn't love Nastasya at all, but he's seeking her hand anyway because of her impressive dowry. Rogozhin only has eyes for Nastasya. Nastasya used to be Tostky's mistress but now she hates the man; she seems to despise Gavrila; she's in a full-on Masochism Tango with Rogozhin, so it's unclear whether she loves or hates him; and she seems to be afraid of Myshkin—his love threatens her self-image as a bad person, and in her more selfless moments she fears that she'll hurt him.
Every Blandings Castle novel, and, in fact, most of the works of P. G. Wodehouse. He was a master at these, and could successfully create and solve one of these plots in a single book. If one were to create a chart detailing the relationships of all the characters who appear in his connected universe, one would probably go insane just from reading it.
The Seven Realms Series has a lot of this, with Han Alistar and Princess Raisa at the center. Raisa and Amon are very much in love with each other, but their love is forbidden. At the same time, Raisa enjoys flirting with Micah Bayar for fun, but their love is also forbidden and it turned out Micah was using magic to get Raisa to like him more, but Micah doesn't seem to care. Han is a supreme Chick Magnet that gets quite a few suitors for himself including Micah's sister, Fiona. Han is a commoner that's managed to piss off the Bayar family by keeping an incredibly powerful MacGuffin out of their hands. ('Cause, y'know, they're evil and would kill Han even if he gave it back.) When Raisa and Han travel to Oden's Ford, Han gets a few suitors from the female student body (much to Fiona's chagrin.) Amon also finds a girl who is, by political standards, a much better match for him. Raisa is not pleased and is only convinced that they won't be together by the magic spell keeping them apart knocking Amon out after they kiss. Then Raisa and Han finally start spending large amounts of time together in tutoring sessions and they quickly start to feel attracted to one another. Then, when Princess Raisa openly admits that as the heir to the throne, she is looking for a Royal Consort, gifts from suitors come in by the boatload. By book three, Raisa and Han, the couple that fans had been shipping for ages finally admitted that they want to be together. Blue Blood politics won't make that so easy, though...
In Maria McCann's As Meat Loves Salt, Jacob loves and marries Caro, but rapes and abandons her over jealousy of her friendship with his brother, Isaiah. Then he meets and falls in love with Ferris, only for Caro to reappear and start an affair with Ferris. Then its revealed that Jacob may have had a sexual relationship with Isaiah in the past.
In Chekhov's The Seagull Medviedenko loves Masha, who loves Treplyov, who loves Nina, who loves Trigorin, who is loved by Irina. Everybody's unhappy.
A poem by Brazilian author Carlos Drummond de Andrade: "João loved Teresa who loved Raimundo who loved Maria who loved Joaquim who loved Lili who loved no one".
A Song of Ice and Fire: Ned Stark married Catelyn Tully to cement the Stark/Tully alliance against the Targaryens, but he also apparently had a thing for Ashara Dayne (who is rumored to be his babymama, and who was also lusted after by Ser Baristan Selmy, and some theorize by Ned's brother, Brandon). Ned also had a healthy amount of Ho Yay with his best friend, Robert Baratheon, who was in love with Ned's sister, Lyanna, but ended up marrying Cersei Lannister. Cersei and maybe Lyanna had the hots for Rhaegar Targaryen, who seemed to reciprocate Lyanna but was already married to Elia Martell, though Cersei is also getting it on with her twin brother Jaime, her cousin Lancel, Lady Taena Merryweather and two of the three Kettleback brothers. Catelyn's sister Lysa is married to Ned and Robert's adoptive father, Jon Arryn, but is also engaged in a clandestine affair with Littlefinger, whom she eventually marries after her husband's death. Littlefinger, however, was madly in love with Catelyn, who loves him like a brother, but no more. Now, though, he's found a suitable replacement in Catelyn's teenage daughter, Sansa. Robert's brother, Renly, is married to Margaery Tyrell, but is implied stated (by Word of God) to be sleeping with her brother Loras. Additionally, Renly's bodyguard, Brianne has a major crush on him, but latter develops a rather tender relationship with none other than Jaime Lannister, Cersei's brother. Yeah, love in Westeros is complicated, and this doesn't even get into Jon and Dany's generation.
Sense and Sensibility: Edward likes Elinor, but never says so. It's because he is secretly engaged to Lucy. Colonel Brandon loves Marrianne who thinks it's kind of creepy. She falls for Willoughby and their attraction is mutual. However, Willoughby marries Miss Gray. Edward's family tries to match him with Miss Morton. Most people ship Elinor with Colonel Brandon but they are just friends. Lucy marries her fiancé's suddenly wealthier brother Robert. Elinor marries Edward and Marrianne marries Brandon.
The point of Pride and Prejudice is resolving one of these. The story presents a family with five sisters who need to get married, otherwise they are be destined to be degraded in society from comfortably-living ladies to spinsters with no money and no good home. Jane hits it off with Mr Bingley whom his family ships with Miss Darcy. Mr Collins briefly pursues Jane until he learns she's going to marry Mr Bingley. Elizabeth is pursued by Mr Wickham, then by Mr Collins, she flirts with Colonel Fitzwilliam and gets proposed by Mr Darcy. Charlotte marries Mr Collins, who first proposed to Elizabeth but was rejected. The Bennets thought Mr Collins might want to marry Mary Bennet instead. Wickham flirts with Elizabeth but also goes after Mary King and then Lydia. Caroline Bingly is after Darcy, whom Lady Catherine ships with her daughter Anne de Bourgh. See the chart.◊
Mansfield Park. Mr. Rushworth is in love with Maria, and Mr. Yates is in love with Julia, but both Bertram sisters are in love with Henry Crawford, who claims to be in love with Fanny, who is in love with Edmund, who is in love with Mary Crawford, who is in love with money.
Emma: Emma ships Harriet and Mr Elton, even though Mr Elton wants her and there is mutual attraction between Harriet and Robert Martin. Mr Elton ends up marrying Augusta Hawkins. Emma finds herself attracted to Frank Churchill, even though he is secretly engaged to Jane Fairfax, and she is totally oblivious of the fact that Mr. Knightley is in love with her but painfully aware that Harriet is in love with Mr Knightley, despite earlier thinking Harriet was in love with Frank Churchill. Mrs Weston, Mr Cole and others also suspect something between Mr Knightley and Jane Fairfax, while Emma suspects something between Jane Fairfax and her foster sister's current husband, Mr Dixon. Nothing gets a Love Dodecahedron going like the out-of-control imaginations of matchmakers.
Persuasion: Mary's husband Charles was originally in love with her sister Anne, who is in love with Captain Wentworth, who flirts with Henrietta (whom Charles Hayter is in love with) and Louisa Musgrove, who eventually marries Captain Benwick, who also showed interest in Anne before Mr. Elliot did, and Anne's sister Elizabeth has always planned to marry Mr. Elliot, but he ultimately runs off with Mrs. Clay, whom Lady Russell and Anne feared had intentions of marrying Sir Walter Elliot.
In Anthony Trollope's Ayala's Angel, the title character is beloved by Tom Tringle, Johnathan Stubbs, and Captain Batsby, but Batsby also takes a fancy to Tom's sister Gertrude, who wants to marry Frank Houston, who is in love with his cousin Imogen Docimer. Meanwhile Lady Albury has an interesting relationship with Stubbs, but is determined not to cheat on her husband Sir Harry. Also Tom's other sister Augusta accuses Ayala of trying to steal her fiancee, the honourable Septimus Traffik.
Let's start with the main character, shall we? Cinderpelt (who seems to have some chemistry with Littlecloud) was in love with Firestar, who never knew. Firestar and Spottedleaf liked each other for the brief period of time that she was alive, and they continue to meet in dreams even after her death, much to the chagrin of Sandstorm, his mate, who previously was friends with Dustpelt, who thought it was something more. Dustpelt eventually became the mate of Ferncloud. Now skip ahead about six books, and you'll find that Cinderpelt has been reincarnated as Cinderheart after her death. Cinderheart falls in love with Lionblaze, Firestar's grandson. Lionblaze used to be 'friends' with Heathertail, who hates him because he broke up with her, and she eventually became mates with Breezepelt.
Tigerstar. First off, Word of God confirmed in an author chat that Leopardstar loved Tigerstar, though he probably didn't feel the same way. He's had children with both Sasha (his true love, according to Erin Hunter) and Goldenflower, and it's questionable if Goldenflower loved him or not.
The New Prophecy is great with this. Stormfur, Ashfur, and Brambleclaw all liked Squirrelflight. Stormfur knew that he didn't have a future with her, and eventually ended up with a Tribe cat named Brook. For a while it seemed as if Brambleclaw and Squirrelflight would become an Official Couple, but fell out because of Brambleclaw wanting to be friends with his half-brother (it makes a bit more sense in context), whom some fans say he had Ho Yay with, and Squirrelflight went to Ashfur. However, she never truly loved him, and later left him for Brambleclaw once more. This drove Ashfur insane, to the point of betraying his Clan, abetting in almost murdering Firestar, and trying to kill Squirrelflight along with her 'kits'. This forced her to admit that they weren't hers, and were in fact the children of Leafpool and Crowfeather. Crowfeather had previously been in love with Feathertail (who died), had a brief affair with Leafpool (which produced Jayfeather, Lionblaze, and Hollyleaf), and then took Nightcloud as a mate in WindClan solely to prove his loyalty and have a kit with her (Breezepelt).
Spiderleg became Daisy's mate, but they broke up after having two kits. Prior to this, Daisy had an unrequited crush on Cloudtail, who was already with Brightheart. Daisy's previous mate was Smoky (who also had kits with Daisy's sister, Floss, who he liked better, according to Daisy) and he fathered Mousewhisker, Hazeltail, and Berrynose. Berrynose hooked up with Honeyfern, but she died. He later mated with Poppyfrost.
Graystripe was Silverstream's mate, and later had kits with Millie. Graystripe's mother, Willowpelt, went through quite a few mates: Tawnyspots, Whitestorm, and Patchpelt (who is her own brother, though that was an accident with the author forgetting they were siblings since, though they have the same parents, they were born in different litters).
Bluestar had a brief relationship with Oakheart and had his kits. Thrushpelt had a crush on her, though she didn't return his feelings, and he pretended to be the father of her kits in order to help her out (since they were the result of a forbidden inter-Clan relationship).
In OOTS, Dovewing fell in love with Tigerheart, but broke up with him when he used her to get herbs and she realized he was training in the Dark Forest. She later became closer to Bumblestripe, and according to Word of God chose him in the end (they just didn't show it in the book because it didn't fit in with the emotional final scenes).
In the prequels, Clear Sky and Bright Stream were mates, Gray Wing had a crush on the latter, while Turtle Tail had a crush on him. After Bright Stream's death, Gray Wing started to have stronger feelings for Storm, who liked Clear Sky, who liked her back, and Storm and Clear Sky became mates. Turtle Tail, feeling hurt that Gray Wing liked Storm, meanwhile, left the group and became mates with Tom briefly before she realized how abusive he was, and had his kits. Storm dies shortly after producing Thunder. Eventually, Gray Wing started to return Turtle Tail's feelings, and they become mates. Meanwhile, Clear Sky is implied to have feelings for Petal, while Acorn Fur (according to Word of God) had a crush on Thunder, who had some Ho Yay with Lightning Tail. Thunder develops feelings for Star Flower. After Turtle Tail's death, Gray Wing moves on to Slate. Star Flower breaks up with Thunder, (who still likes her), and gets together with Clear Sky, (his father) instead. Thunder eventually moves on to Violet Dawn. Damn.
A slight invocation of this trope appears in The Three Worlds Cycle, specifically in the second quartet to make up the cycle. Nish loves Tiaan, who doesn't love him back. Then Nish has a short affair with Irisis, who hates Tiaan, simply to get back at her for rejecting him. Then Irisis has a brief affair with Xervish Flydd, who seems to have possibly had a thing with scrutator Halie. Meanwhile, Tiaan falls in love with Minis, who loves her back but betrays her and spends three books beating himself up over it. Tiaan then meets Gilhaelith who seems to lust after her. Back to Nish and Irisis, they end up in a will they/won't they situation right up until the end of the fourth book, where Tiaan swears off men for life, Flydd has been castrated anyway, and Irisis and Nish finally get it on. Well, almost manage to...
In The Mortal Instruments, Clary falls for Jace. Long time best friend Simon has always loved Clary, but Clary doesn't find out until later. Alec has a crush on Jace, who doesn't find out at all. Simon has a crush on Isabelle, Alec's sister. She loathes him when they first meet but he grows on her. By the end of City of Bones, Jace has fallen for Clary as well. Later, Maia is introduced. She likes Simon, and he likes her back. But we can't let anything in this story be simple. Sorry Simon.. By now, Magnus Bane has been introduced. Guess who ends up with him? In City of Glass, yet another player is introduced, Sebastian. Clary has a short fling with him. Only it turns out that Jace and Clary aren't actually related. But in Sebastian aka Jonathan's case, it's for real. And he knew it. Then there's Jocelyn's love history, which is simpler (Lucian / Valentine), or other side love interests like Isabelle's fling with Meliorn or Maia's ex Jordan. As of City of Lost Souls, Clary/Jace is on with difficulties, Alec/Magnus is off, Jocelyn/Lucian is on, and Maia/Jordan is on. As of City of Heavenly FireClary/Jace is still on, Alec/Magnus is back on, Jocelyn/Luke are married, Maia is with Bat, who she had a thing with before the start of the series but broke it off with because she wasn't ready to be in a relationship so soon after Jordan, who is now dead, and Simon/Isabelle were on, until Simon gave up his immortality and all his memories of the Shadow World, and his friends connected to it, to Magnus' father to get them all out of Edom after the death of Sebastian, who had an affair with the Seelie Queen.
In Ranger's Apprentice Halt spends time with Alyss and thinks If I were twenty years younger... He ends up marrying Lady Pauline. Alyss kisses Will. Will spends time with Evanlyn but can't handle that she's really Princess Cassandra and falls for Alyss instead. He and Evanlyn still have feelings for each other and this causes tension between Alyss and Evalyn and between Will and Horace. Horace and Evanlyn end up falling in love and become engaged, as do Will and Alyss.
Laurell K. Hamilton's Anita Blake is an infamous example of a Love Dodecahedron, it'd be easier to name a character not in love with or part of Anita's Reverse Harem.
In Prized by Caragh O'Brien, Leon loves Gaia, who is also loved by Chardo Will and Chardo Peter, who once dated Adele, who is now married to someone else.
The Count of Monte Cristo is full of this. To start off, Edmond Dantes is in love with Mercedes who however marries Fernand, who betrayed Edmond and had him thrown into the Chateau d'If. Their son is supposed to marry Eugenie, daughter of Danglars, who was one of the people Fernand put together to betray him. That's not possible, because Edmond's slave Haydée tells him about how Fernand betrayed her father. So, Danglars' wife, who had a scandalous relationship with Villefort, another of Edmond's betrayers, had a son, Benedetto, with him who is also supposed to marry Eugenie, but she leaves in a supposedly lesbian relationship. Villefort's daugther Valentine loves Maximilien Morrel but she is forced to marry Count d'Epinay, which is finally not possible because Valentine's grandfather killed d'Epinays grandfather. Oh, and Edmond suggests that Morrel marry Haydée, but in the end marries her himself. See spoilers  for the full details.
In the Star Trek Novel Verse, Captain Klag reads a novel involving Damiani (a three-gendered species) and Andorians (four genders) in a dangerous romantic septangle.
Legacy of the Dragokin: A weird example in that it's not the focus of the plot and yet it is at the same time. What is definite is that it is NOT played for comedy.
Daniar and Kalak are married but having problems so they both go to comfort from other people (Jago and Zarracka respectively) while Mordak has a Lady and Knight thing with Zarracka and a yandere thing for Kalak while both of them can't stand it/him and yet are dependent on Mordak, who, by the way, is weakened by love. Then add in the fact that Kalak is jealous of the attention his wife gives their son who is head-over-heels for his mother's protegee who is Married to the Job and it gets more complicated.
In Sherwood Smith's Inda series, the people involved keep shifting as they get killed or find true love, but at around the midpoint of the story: Hadand is in love with Evred, who is in love with Inda. Hadand is betrothed to Evred in a political Arranged Marriage (which he inherited from his brother who Hadand was originally betrothed to). Inda is still in love with his original arranged bride, Tdor, but with the death of his older brother he will inherit his brother's betrothed, Joret, who is in love with Cama, who loves her back but is betrothed to somebody else entirely. Tdor still loves Inda, but is now betrothed to Whipstick Noth, who doesn't have any kind of strong romantic feelings for anybody but likes Tdor well enough.
In The Great Gatsby, Gatsby has his heart set on Daisy who is married to Tom, who has an open affair with Myrtle, who herself is married to George.
In Beautiful Darkness. Ethan is in love with Lena, who he believes left him for John Breed, and is beginning to be attracted to Olivia. Link is also attracted to Olivia, but at the same time he's still pining for Ridley. Ridley and John are often seen together, so subtext could lead to interpretation of them as a couple. Lila Jane Evers' first love was Macon Ravenwood, but he had to leave her because he was afraid of hurting her, so she met Ethan's father. She's still enough in love with Macon to save him from dying. He never stopped loving her.
In New Arcana, Roland likes Kiri but hooks up with Miranda, who has a crush on Luke, who has a crush on Corryn, who used to be involved with Kurt but also seems occasionally to hook up with Elmin, who hooks up with anyone, including Kiri and Miranda, who ends up in a relationship with Wynne, who is an “old friend” of Elmin’s.
When A Man's A Man, by Harold Wright: note spoilers shown because it's in the public domain Phil loves Kitty, who loves Patches (who is temporarily engaged to her) and briefly deludes herself into loving Professor Parkhill (who proposes to her, but is rejected). Patches himself loves Helen, who is Happily Married to Stanford. At the last minute, Kitty conveniently realizes that she really loves Phil, and the two get together. The Professor leaves the ranch, and Patches goes on a journey with Yavapai Joe, who he has homoerotic subtext with.
All Men of Genius: The Duke loves Violet/"Ashton" (not knowing that they're the same person, and emphasising the former while downplaying the latter since it would be unprofessional and he doesn't consider himself gay anyway). Violet reciprocates. Cecily thinks she loves "Ashton", though eventually decides that she doesn't. Violet's friend Jack and nasty blackmailer Volio both love Cecily, though Cecily initially dismisses the former and doesn't even know about the latter (who thinks he's had a long and passionate correspondence with her, but has actually been conned as revenge for trying to blackmail Cecily's governess).
Gone with the Wind: Scarlett loves Ashley, who is sexually attracted to her, but loves his wife, Melanie. Melanie loves Ashley as a husband, she loves Scarlett as a sister/best friend, and loves/likes Rhett as a friend and as a hero who helped her during the War. Rhett loves Scarlett, contemns Ashley, and loves/admires Melanie platonically as a great lady.
Chronicles of the Kencyrath: When the previous generation was of courting age, they were a mess of this. Ganth and Trishien liked each other, but Ganth's father Parental Marriage Vetoed it. Ganth's brother Greshan liked Rawneth—who didn't really like him so much as she wanted to marry him for political power, but they suited each other very well anyways. Greshan's grandma Grandparental Marriage Vetoed it. Their father tried to marry Ganth to Rawneth, but that didn't really go anywhere. Greshan was killed, and Rawneth tried to raise him from the dead, and even got his father to approve their marriage under those conditions. But she ended up getting Bed Tricked by a different undead. Meanwhile, Ganth fell in Love at First Sight with Jamethiel, and next to her, Trishien couldn't hold up. Jamethiel eventually became his consort, but was her brother's consort for 3,000 years before that. Also, their half-brother Tirandys loved Jamethiel too, but was only ever an Unrequited Tragic Maiden in that regard. Jamethiel's own feelings for any of them are highly ambiguous.
The current generation is really pretty dodecahedronal too—just that it appears in small enough pieces it doesn't seem so tangled. But here it goes: Hatch is completely in love with his cousin Prid, who we think kind of likes him back, but is also enamored with (and Shotgun married to) Jame—but Prid and Hatch are from a polygamous society so it's ok. Jame sees Prid as an Unwanted Spouse. Timmon also likes Jame, and Jame kind of likes him back. Narsa is enamored with Timmon, and Timmon's sleeping with her but only sees her as fun for now—also, he's trying to make Jame jealous. Narsa thinks there's a Betty and Veronica love triangle going on, with Jame as the exotic, mysterious, unattainable Veronica, who distracts Timmon from her, the unexceptional Betty who really loves him. That's really not what's going on—Narsa was only ever a substitute, never a love interest. But Narsa thinks it is and she tries to Murder the Hypotenuse in her desperation—though Jame told Narsa she could have Timmon, because while Jame kind of likes Timmon, she definitely loves her twin brother Torisen more. Torisen loves her too, but they're locked in UST for the time being, because of a lot of emotional baggage. And if we're mentioning Torisen, then there's Kallystine—his ex, who seduced him even though he hated her, maybe using rape-y Love Potion stuff. So after that, Torisen's first reaction to liking someone is to hate and fear his own attraction.
In M.L. Rio's If We Were Villains, all the main characters except Filippa are embroiled in one. Meredith and Richard are Sickeningly Sweethearts (everyone calls them "Meredick"), but Oliver is in love with Meredith and James. After a horrific fight with Richard, Meredith reciprocates. Meanwhile, James is in love with Wren and Oliver.