Oh jeez, this is a disaster in the making. I'm not judging on an idea of a threesome - each to his own. But a threesome where two are attracted and the boyfriend is already jealous is just crazy.
The most important boundary with a threesome is making sure that each participant is doing it for the right reasons. This includes believing that it's a good idea and not being pressured by someone else. The fact that you don't think it's a good idea and have been pressured straight up mean that it's wrong.
You would be best to look at the issue of being attracted to the mate. Either end it with the mate or end it with your boyfriend. But don't do this.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.