How to start a relationship with Holly? I got to the point I had sex with her in the fair trip and then the game forces Ian to stop doing anything with her (no choice between her or Lena).
If, when talking to Jeremy at Shine, you say you are romantically interested in Lena the games locks you in that route and closes Holly's for Ian, i think.
And damn, I have a problem, because in chapter 12, while 3 is talking on the beach, Holly says she's bothered you enough. And Ian replies that he hasn't, but he'd like to talk to Lena alone. And Holly leaves, and 2 talks about Cindy, so maybe Ian can't cheat on her with Cindy so he can have a threesome with Holly?
The Trinity route doesn't allow for cheating after committing to a relationship as far as I'm aware, haven't tried it. Holly's route with Ian certainly doesn't.
Basically,
The main determinant to get the Trinity route is Lena's attraction to Holly plus Ian has to be in a relationship with one of them. I don't think the Lena+Holly route allows for the throuple, could be wrong. The easiest way is be on the Ian+Lena route and, on the first time you can choose who Lena masturbates to, choose Holly. From then on always pick the choice to heighten her desire for Holly, such as kissing her on the girl's hangout at home (either Lena's or Ivy's, depending if Holly is in the gym) and trying to kiss her after committing to the relationship with Ian. If you are doing everything right you should notice that Holly gets progressively more nervous around Lena until ch 11.
Holly going to the gym or not should only affect how outgoing she is as long as Ivy doesn't sink her claws into her and Lena always intervenes when Ivy pushes Holly to get it on with Mark.
Other than that, if going through the Ian+Lena path, I find that going to the book fair with Holly makes the most compelling storyline, as it shows that both him and Lena are interested in her relatively early on (be sure to stop at kissing though).