I lost my virginity in a threesome but it was a VERY different situation to yours. It was a drunken casual sex encounter with a girl i hit it off with at a student party and her boyfriend who i thought was cute. none of us thought through the emotional consequences, at the time all i knew was that they consequently spilt and never spoke to me again
bad times, tbh that upset me a lot more than the unspectacular sex(and reinforced asexy path I walked between then and finding out bout poly). I think I value sex alot less than many, i slept with them because i was curious and atracted and tbh for me it felt like less pressure, but chances are i'd have lost my virginity to a stranger in any event because of my level of dev at time (i'm a bit slow
) I'm glad you're more considered than I was, you say you have some issues arround sex imo all the more reason to take it slow, though also want to say that you're very unlikely to be alone in whatever issue, despite how it feels. I also wondered if you find her sexually atractive? or is most of the sexy energy with him? IMO a threesome would only be a good idea if you think it would be comfortable.
Peace and love