Mark has had plenty of girls (and some guys) show obvious interest in him, but he's only ever returned the affection given to him by Natalie and Katy... until Sharon enters the picture, at which point he drops both of them like a bad habit and hooks up with her.
Downplayed: Sharon is a Chekhov's Gunman, who's been a peripheral acquaintance of Mark's for some time. He only starts paying attention to her when he realizes that this Love Triangle situation is toxic and he doesn't really want either of them.
Sharon's personality combines the best of both worlds.
Mark was already beginning to wonder what he saw in either of them when he met Sharon, and felt something for her he'd never felt for the others.
He's more compatible with Sharon than Natalie or Katy.
Averted: The love triangle never has more than two girls.
Enforced: The writers are genuinely scared of the Shipping Wars that would break out if they ever resolved the Love Triangle; Sharon is introduced in the hope that she'll be better received.
Lampshaded: "Let me get this straight - you have two girls fighting over you, and you fix that by going and getting a whole new girl. Man, I wish I had your problems, Mark."
Invoked: Sharon sees Natalie and Katy fighting over Mark all the time and decides to try and see what's so great about an ordinary guy like him.
Exploited: Mark stages an Operation Jealousy with his new friend Sharon as a way to find out how Natalie and Katy react.
Defied: Sharon decides she doesn't want to get into a love quadrangle and decides to pursue another guy.
Discussed: "You know, Natalie, you seem awfully sure that if Mark doesn't choose you he'll pick Katy. There are other girls in the world."
Conversed: "Mark will never choose either of those two. There'll be a new girl introduced 3 episodes before the finale, mark my words".
Implied: The work ends with No Romantic Resolution, as Mark gets on a bus. As the bus pulls away, he starts chatting and laughing with a beautiful woman sitting in the next seat.
Deconstructed: Mark chooses Sharon because he's tired of Natalie and Katy warring over him, but it turns out he and Sharon aren't a particularly good match. And Natalie and Katy both feel so betrayed by Mark dropping them for Sharon that he loses their friendship entirely.
Reconstructed: Mark's new friend Sharon points out to him that Natalie and Katy have actually been pretty unpleasant to him lately, and he'd been so wrapped up in trying to choose between them that he'd forgotten about anyone else. She encourages him think about what it is he really wants, and look for a girl who gives him that. He plays the field a bit, but his friendship with Sharon only deepens, and he comes to realize that she's the one for him.
Played For Laughs: Natalie and Katy fight over Mark, until Sharon comes around ... and Natalie and Katy start to fight over her, completely forgetting about Mark.
Played For Drama: Natalie distrusts Sharon and gives her If You Ever Do Anything To Hurt Him speech in front of several witnesses. Katy is so obsessed with Mark (or with the large sum of money he's going to inherit very soon) that she plans to kill Sharon and frame Natalie for that crime. One day Sharon (who prior to that did do something that could hurt Mark) is found dead, and the main suspect is... Mark. He takes the blame and goes to jail to protect the woman he loves - the one he believes is the real murderer.