It's a fetish in the purest sense of the term. Most people do know that a classical three way relationship can't work, our brain just aren't wired that way
However, as a fetish, it's very hot, because it opens many domination/submission scenarios (which are the basis of most fetishes tb'h) and also more creative sex scenes
I don't have a bf, but if I had one and he were up for it, I really wouldn't mind trying a three ways at least once. I think however it would kinda suck for the third lad.