AudreyWinter: I might have mixed up Yennefer and Philippa. But in any Case, Triss and Yennefer are very close and very good friends.
Why does Geralt keep coming back for Yennefer? Maybe you've heard of it: love. Those two might not be able to make it work because of their different views and habits, but they also can't live without one another, they truly suffer when they are apart.
As for Triss, we should be fair to her. She wasn't portrayed very true to her book self in the first game, to me she appeared a lot more scheming in the game. In the books she's this hopeless do gooder who wants to help everyone ("don't let Merigold get to the witcher's secrets, she might want to use them to heal everyone").
And she's simply been in love with Geralt from the moment she first met him and still is. She would do anything for him. Even sacrifice her own happyness if it means, Geralt can be happy with Yennefer. Give the tragic redhead some credit, please. She's not that sly person you make her out to be.
TW2 girly Triss doesn't do her justice either, though. But I like her line in the Elven bath: "I owe you that, I owe HER that." That one line redeems game Triss for me. :)
Also, if you're living long enough and you're living outside the usual rules of society anyway, you have no need for lables like heteroxesual, homosexual, bisexual. Labels ar for cans, not people. ;)
Haven't read enough to make an informed post. But from a few that i have read (I think it was Blood of Elves) she didn't really fall in love with she first met him. She started out by being curious about what Yen's guy was like, and during one particular Yen-Geralt fight, she sort of used spell to seduce Geralt. After the sex, she felt something new in her, and for a while she just couldn't put her finger on the new sensation, and that is probably when she started having feelings for Geralt, so to speak.
So far from what I've read, she seems to be more frank and more do-no-harm person than Yennefer. Yennefer was more of a schemer and a bitch (for lack of a better word). As to sacrificing her own happiness for Yen, I think she is still a bit undecided, as in she finds it hard to let go, at least while she was in Kaer Mohen with Geralt she seems to try to be intimate with him.
But I haven't read that much about Triss really :(