Ok so I've always been straight but recently I think I'm getting feelings for a woman. She's become a very close friend, I think about her all the time, she gives me butterflies, I'd love to kiss her and touch her but the thought of touching her downstairs doesn't do anything for me. Not into that. Although I guess I would be willing I suppose. Why am I having these feelings about her. She talks to me about her boyfriend wanting a threesome all the time, she even said that she says my name out loud to him while they have sex. I asked her the next day If she remembered everything we spoke about and she panicked saying we didn't kiss did we did I kiss you... why would she think that? I'm just so confused she's all I can think about. Do I tell her how I feel? I don't think it would change anything because we are so close. I don't want a relationship at all, I want fun with her and her boyfriend and she's put that in my head but I can't talk about it.

Maybe the fact you cannot talk about it is the problem. You cannot share anything, not even a bed without TALKING about it, and expressing yourself.

Maybe try a note or letter. Of course you just may not be ready for such an experiment in this sort of relationship despite the lusty titillating thrill that is going on in your own head. How old are you and do you, or have you had intimate relationships before? How long have you known this couple?

If being a bit scared makes you cautious, that's not a bad thing. Maybe thinking this through without the fantasy or lusty suggestions is a better approach.

First of all, try to accept your thoughts and feelings. They are what they are. You aren't thinking about hurting puppies or children. You have a crush on a woman despite being straight, or thinking of yourself as straight, it's all the same.

Many people, myself included, believe that 'gender preference' moves along a continuum, a fancy way of saying it can vary and it can change and it can change back and so on!
You can make this decision. If you need to get it out, do so. If you need to wait, or never tell her, do that.

Acting on what is in your mind is something else that moves along a continuum. Obviously we don't act on everything, like running that driver who cut you off off the road. You have options.
Talk about it with her
Don't talk about it with her
Plan it for X time from now, and decide then
Talk to another trusted friend
Talk to a therapist (if you are so conflicted that doing nothing is ruining your life)

Even if you talk about it with her doesn't mean anything has to happen. That's the approach I would take. No harm in it at all.
How she feels about you needs to be clear before you do anything physical, because there's a third party involved - her boyfriend.
If you are attracted to the idea of both of them, fine. If just her, then you could get involved in something you didn't want.
Be VERY clear with each other before jumping into a 3some.