Originally Posted by Nighthorse
If I gave advice, it would be to explore that relationship with her first, or else it may only be just a "sex thing", and probably not the best kind of sex. If I were in your shoes, I'd be up front and ask her if she'd consider becoming romantically involved with me on a non-exclusive basis.
I disagree with this advice.
She's your friend and you have feelings for her, but you have a boyfriend. Your boyfriend and you have not discussed a polyamorous lifestyle.
To go exploring a polyamorous lifestyle with someone else before talking to your partner is dishonest to both your partner and the friend.
It's dishonest to your partner because it amounts to exploring an affair.
It's dishonest to your friend because if she feels the same way about you, you won't be in a position to explore those feelings without betraying your boyfriend.
My advice is to talk to your boyfriend about polyamory and tell him that you identify with this lifestyle. Depending on how he reacts, tell him that you believe you have feelings for your best friend but that you haven't taken any steps towards acting on those feelings. Ask him if he would support you exploring those feelings as part of what you need to prepare for the threesome, in the sense that you believe having sex with your friend will bring up feelings that you won't later be able to just put aside.
It's also possible that your friend will not want a threesome with you and your boyfriend, so make sure he knows that you're not in a position to guarantee that outcome. Just because she's pansexual does not mean that she's attracted to him, nor that she likes threesomes. I identify as pansexual and poly, but I only like to be in bed with one sexual partner at a time. Pansexual means that you don't exclude any partners on the sole basis of gender; it does not mean that you want to have sex with everyone.