I couldn't do it inside of a relationship- outside maybe but god it would bring up so many relationship issues... My ex used to want to have a threesome with another guy and I hated any thought of it. I wouldn't even feel comfortable if it was another girl I was having sex with either it would just feel so wrong.
I just can't recommend it at all in your case OP. If you and your partner eventually break up and you are considering it in a future situation then I would say go for it right away with no reservations.
