This sounds like a setup for a lot of fighting. From how it sounds, though, I'm not sure if you mean a poly relationship or just a threesome. Here's how it sounds to me: Either way, you need to give a lot of thought to your best friend. That is, you seem to care about your best friend as a friend but not in a boyfriend way. He wants to be with you, but you obviously don't want to be with him (otherwise you'd be in a relationship, right?). So you figure that you can make him happy by having a poly relationship or at least a threesome so he can finally just be happy. At the same time, you don't want your boyfriend to be hurt. So what do you do? Sounds like your boyfriend is humoring you and being pretty nice about it. What exactly do you picture happening after you start sleeping with your best friend? Do you think your friend will continue to be willing to share with your boyfriend? Do you think you'll be able to continue having a friendship with your friend-turned-boyfriend? Are you sexually attracted to your friend? Because you need to really worry about all of these factors.