Threesomes

My man has a fantasy of having a threesome. While its not my fantasy this is something i want to do for him ti fulfill that. I more so would like it to be a one and done type of thing where as he would like to have it be once in a blue and not just a one time thing. Im not really sure how im feeling about this i have mixed emotions. As much as i want to do this for him idk if im going to be ok seeing another woman touch my man. Hes stated many times this is just a sexual fantasy, theres no emotional attachment at all.

Has anyone had any experience with this good or bad? What are your thoughts?