Unless Sabrina has only slept with OP, there is high likelihood she's been in some contact with someone else she's fucked in her past. I don't see him freaking the fuck out about that. The problem I see here is Sabrina. She's not entitled to information about his past what so ever, so long has his behavior with her once they are together isn't negatively impacted by that past. So if he can be friends without letting the romantic shit linger in a disrespectful way to Sabrina it's really not her business where OPs dick once went. Now I think OP was generous by offering the full truth, he wasn't obligated to do so but he wanted to not have omissions between them and that's a good thing. The fact Sabrina can't handle OPs truth is again her fuck up not his, not Jake not Lena. I'm my opinion he should give Sabrina space, let her decide if her insecurities about him having friends with an intimate past, that he's willing to be honest about with her, is a deal breaker. If she can't handle who OP is she either 1 shouldn't ask and 2 shouldnt try to make him feel bad about her fear the he'll prefer them to her. Look how OP writes about all of them, he adores Sabrina and if she can't see that she's blinded by her own insecurity. He's hurt bad by losing these two close friends, and Sabrina's insecurity and jealousy is the root of that pain. The problem isn't anyone here but Sabrina.

My opinion, let her go, keep the friends. If she comes back maybe you all can set some very loose ground rules for a little while that let Sabrina feel like the past is the past and she doesn't need to worry about an intimate future between the friends. Or they should just fucking include her too and make a big happy intimate friendship group. What OP is suggested though, that's the thought process of someone that's been manipulated by the insecurities of another. I repeat, Sabrina is the only problem here. It doesn't sound like anyone else did anything wrong. Their past together makes one person uncomfortable so now all three of them have to suffer? F Sabrina.