Cheryl Blossom is a third rival, often portrayed as a selfish and spoiled seducer. Her presence is often a factor that makes Betty and Veronica — temporarily — call a truce.
There's a second traditional love triangle in the comics; Veronica as the Archie must choose between the reliable nice-guy Archie (the Betty) and the bad boy Reggie (the Veronica to Veronica). This plays off the main love triangle by giving Veronica another option, emphasizing her hard-to-get, mercurial nature. Poor Betty seldom gets to the play the Archie because having another love interest would take away from her reliable-but-slightly-dull availability.
And even in New52, this dynamic seems to continue in Grayson. Helena is Dick's partner in Spyral, while he and Barbara still appear to have lingering feelings for one another, despite the latter thinking that Dick is dead.
Also Starfire (Veronica) and Barbara (Betty). The first was an extraterrestrial princess descended from cats who had spent her adolescence in slavery. The second was the daughter (or niece and/or adopted daughter) of Gotham police commissioner James Gordon who had spent her adolescence earning a library science degree. Kory was emotionally open and naïve about life on Earth; Barbara was all about knowledge. Kory was a towering, buxom, barely-dressed supermodel who represented physical power and pleasure. Barbara at that point was paralyzed from the waist down and represented intellectual power and pleasure.
Tim Drake (3rd Robin) from the Batman series has had his own personal love triangle in the early run of his solo book. He was dating civilian Ariana Dzerchenko (Betty) but was attracted to vigilante Stephanie Brown (Veronica) at the same time. Ironic, because Ariana has Veronica's physical appearance and bitchiness, and Stephanie has Betty's physical appearance and tomboyishness. Ultimately, Stephanie wins the triangle, with her becoming his most high-profile (and longest) relationship in the series.
Even after Stephanie and Tim's breakup, her fake 'death', Tim dating a new girl called Zoanne and later Stephanie's subsequent return to Gotham — Stephanie was still the Veronica, this time to Zoanne's Betty. And again, Stephanie wins the triangle (technically), with Tim breaking up with Zoanne on the phone, and then calling up Stephanie to try and patch things up with her.
Any of Tim's civilian girlfriends will end up being the 'Betty' by default due to his working partnership/love-hate-on-off relationship with Stephanie (who is always the Veronica).
In Silver Age Superman, Lois Lane was somewhat Veronica-ish compared to Clark's childhood sweetheart Lana Lang. In the early modern continuity, Lois, a brassy investigative journalist who wouldn't give Clark the time of day romantically, was the Veronica, while sensitive, feminine, openly-in-love-with-Clark Cat Grant was clearly the Betty.
Reversed with Lois (now portrayed a sensible no-nonsense reporter) and Cat (vain and demanding gossip columnist) as respectively the Betty and the Veronica in Superman: Secret Origin, which once again re-established Superman's origin in the post-Infinite Crisis continuity.
And later, after Lois fell in love with Clark and discovered his secret identity, she viewed Wonder Woman as the Veronica on several occasions. This was an unfounded worry on her part though, since Supes and Wondy are Like Brother and Sister... although not in the New 52.
Speaking of Superboy (Conner Kent) and Robin III (Tim Drake), both have dated Wondergirl. It wasn't much of a triangle to begin with, but Tim could be Betty/the nice guy and Conner Veronica/the bad boy. With the new Teen Titans reboot, it seems like there's a possibility this triangle might be happening soon.
The Deadpool: Merc with a Mouth mini-series had a literal Betty (Swanson, an A.I.M. Hot Scientist who is disgusted by him) and Veronica (Chase, an ESU professor who had the hots for him). Unfortunately, the latter ended up being zombified, so he's stuck with the hot blonde who hates his guts.
In Fantastic Four, Namor is the Veronica to the Invisible Woman]'s Archie and Mr Fantastic's Betty.
And as of Dark Reign during which Namor joined the X-Men he's become the Veronica to Emma Frost/Archie and Cyclops/Betty.
The early nineties Namor series had a love triangle with Namor as Archie, Carrie Alexander (who helped him set up Oracle Inc. and her and her father cured Namor's blood chemistry imbalance) as the Betty and Phoebe Marrs (Rich Bitch who ran Marrs Corp with her brother, the two of them tried to ruin Oracle Inc. before Pheobe legimately fell for Namor) as Veronica.
Incredible Hulk has Betty choosing between Bruce the Betty and Glenn the Veronica.
The Chaos War would also have Jarella as the Betty and Red She-Hulk aka Betty as the Veronica.
A lot of the pairings have been seen as analogues to the Archie Comics' Betty and Veronica, especially Gwen Stacy and Mary Jane Watson, even though the latter is a redhead, not a brunette. Gwen seemed to have won...until she died. Later, the comic briefly had Mary Jane as the Betty and Black Cat as the Veronica. Then Spidey hitched up with MJ and spent the next twenty years of real time in marital bliss with an understanding supermodel... ahem... nothing happened.
Spider-Man writers seem to be fond of this trope. Originally it was a choice between "mature" brunette secretary Betty Brant (amusingly, the Betty) and the more girlish, flighty blonde Liz Allen (Veronica), who was briefly relieved by rich, stuck-up Gwen Stacy. After Betty left, Gwen became the sweet Betty and the Veronica role went to MJ. After Gwen's death MJ became the Betty and later on the Black Cat became the Veronica. Mary Jane in some ways subverted the Veronica role, though, as she always got on well with Peter's Aunt May (better than Gwen did, in fact) and always was the surrogate parents' choice.
The popularity of the trope manifests itself in the way characters are reinterpreted by readers so that they resemple the Archie archetypes more. This happens especially with Gwen Stacy — many of her fans incorrectly see her as a Girl Next Door like Betty Cooper (she comes from an upper-class Anglo-American family and Peter Parker only got to meet her when they started college because she lived in a different part of New York) and tend to downplay her beauty (Gwen was introduced as a high-school beauty queen and in one story she is taken along to the Savage Land to provide bikini-clad eye-candy). This popular misconception probably contributed to Gwen's reinvention in various adaptations, where she for instance becomes Peter Parker's high school classmate.
Ben Reilly had the spoiled and sometimes selfish blonde fashion student and the twisted, down to earth gothic girl. She was also the daughter of uncle Ben's murderer. In effect he had two Veronicas.
The Ultimate Universe swaps them, making Mary Jane the Betty and Gwen the Veronica. Then Peter chooses Mary Jane and Gwen is killed. Peter subsequently got involved with Ultimate Kitty Pryde and the clone of Gwen, but always returned to Mary Jane, until he was ultimately killed. Although the three alternated between being part of an Official Couple and the Rival, in truth all three are more Bettys than Veronicas.
Possibly in Thor's case, Sif could to be his "Betty" and Amorathe Enchantress his "Veronica" or Jane Foster is Betty and Sif is Veronica.
X-Men had a long-running Love Triangle with Jean Grey, Cyclops (Betty), and Wolverine (Veronica)...which was turned on its head during Grant Morrison's early 2000s run, which introduced a Betty And Veronica love triangle with Cyclops as the center figure, Jean as the Betty, and Emma Frost as the Veronica. Then Jean dies, with the writers claiming she won't be back, but absolutely no readers believe that since coming Back from the Dead is part of Jean's power set.
In the earlier days, the Jean Grey love triangle had Angel in place of Wolverine, as Wolverine hadn't been introduced yet.
Another example was the love triangle between Havok, Polaris, and Iceman with Havok as the Veronica and Iceman as the Betty. Havok and Polaris have been through hell and back together since the earliest days of the comic.
Taking Rogue as Archie, she has Joseph (Betty) and Gambit (Veronica), then she has Gambit as the Betty and Magneto as the Veronica.
All-New X-Men was on track to set up yet another Betty and Veronica relationship involving Cyclops, this time taking full advantage of Rule 63 and Time Travel, with Teen!Scott matched up with Teen!Jean (Betty) and X-23 (Veronica). But then Scott goes into space and the Love Triangle is shot down completely before it can get off the ground.
Another X-Men example is Generation X, where Bald Black Leader Guy Everett's affections was one of many things spunky Jubilee (Betty) and beautiful, classy Monet (Veronica) butted heads over.
The original Captain Marvel series had the hero torn between his regular, dependable fellow Kree military nurse girlfriend Una (Betty) and a female security officer at the NASA facility he was infiltrating, sexier and more assertive ex-military career girl Carol Danvers (Veronica). Though the stories also played around more than a bit with the stereotypes; for example, the Fragile Flower Una was actually the one of the two who usually showed more coolness and competence under fire, and Marvel by and large stuck to her, even when Carol pursued him quite aggressively by the standards of the day. And then Una got the unhappy ending, while Carol got to be a superheroine herself.
British 1940's comic strip Buck Ryan had private investigator Buck's action secretary Zola (blond) as Betty, and his originally arch-nemesis Twilight (brunette, missing an ear) as Veronica.
Harvey Kurtzman brutally parodied the Archie love triangle in Starchie in MAD where he basically sees this dilemma as an example of a shallow, feckless teenage kid who manipulates the emotions of his Love Interest, and strings along women into making him a prize so he can exploit and use both of them. In this parody, Starchie ends up in jail and laments how he poorly treated Betty and cries when he hears that he got over him and is now Happily Married.
Lanfeust is a textbook example with the hero's fiancée Ci'an (the Betty) and her troublesome yet alluring sister Cixi (the Veronica). The two are sisters: gentle, proper Ci'an packs a mean right when angered, while sex-obsessed Cixi sometimes shows a softer heart than her sister, sometimes inverting their role. He ends up with Cixi after she becomes awesome. Well, even more awesome. Lanfeust and Ci'an actually split up before that due to Ci'an falling for the finally grown-up Or-Azur while Lanfeust realized he couldn't provide her with the stable life she aspired to.
Minimonsters has Victor Von Piro (Veronica) and Frank Einstein Jr.(Betty). However, the Love Triangle thing is subverted: Piruja (The not-so-Cute Witch version of Archie) is extremely amorous; and she was in love with Franky until she met Victor. As a consequence, she fell in love with him inmediately. In both cases, her love is unrequited.
In Seconds, Katie's choice between the stable and romantic Max (Betty) and the more passionate and sexual Andrew (Veronica), as well as her professionalism with these relationships, is a major plot thread that is the catalyst of the story and influences several revisions.
Sin City has Dwight as Archie with badass prostitute Gail as Veronica and sweetheart waitress Shellie as Betty.
Sleeper has a version of this with the main character Holden Carver, his former government coworker Veronica (Betty), and supervillain Gretchen/Miss Misery (Veronica). The end result is a truly spectacular and accidental Murder the Hypotenuse, with Veronica shooting Gretchen, thinking she was about to kill Holden, and Holden reflexively shooting Veronica dead. He then had an experimental surgery to cure his condition that left him in a vegetative state, with his superior creating a little mental world for him where he lives on a beach with both Veronica and Gretchen, making this a rare canononical quasi-OT3 solution.
Being an Archie Comic, not even Sonic the Hedgehog was safe from this. To wit, Sonic and Princess Sally were always the Archie and the Betty, flip-flopping with the long list of Veronicas that wandered in - Bunnie Rabbot (who hopped off that train the same panel she hopped on), Geoffrey St. John, Mina Mongoose, Fiona Fox and Monkey Khan. Amy Rose was usually the Third-Option Love Interest when Sonic was the Archie.
Fiona Fox briefly had this between Sonic and his Mirror UniverseEvil Twin Scourge. She decided that Scourge was a lot better than Sonic. Unwittingly, she caused this between Sonic and Tails, mostly because Sonic had no idea Tails had any sort of attraction towards her and after it initially came out, Sonic thought it was just water under the bridge.
Spy Boy: Spygirl and Bombshell. Even their fake identities in Alex's high school parody this.
Conversed somewhere in a Vampirella issue, where a critic called Vampirella and Pantha "...Betty and Veronica with fangs...". Square Peg, Rhombical Trope - they fit the definitions given here only approximately.
Zot! Jenny is torn between Zot (Veronica) and Woody (Betty). Unusually, both acknowledge that it's Jenny's choice and they genuinely like each other. Also, Zot's blonde, while Woody's brown-haired.