This sucks

I’ve got a group of friends at work, one of which I sleep with named Jared (no strings attached) and two of them used to date, rob and Sarah. Rob and Sarah broke up a few months ago and are still really good friends but Sarah still has feelings for him. Now here’s the problem: Rob wants to fuck me. He wanted to have a threesome with me and Jared while he was with Sarah and I said I’d love to if Sarah was okay with it but he said he wanted to keep it a secret from her so I said no. Now they’ve broken up he still wants to join me and Jared and I high key want him to but I just can’t do that to Sarah.

Idk it just sucks. I know I should have told Sarah when they first asked for a threesome but I didn’t want to fuck up their relationship and I didn’t want to make things weird with me and rob. Now if I tell her I think it’d just hurt her.

Idk.

Any thoughts? Advice?