yeah. it has worked well for some cultures / societies.

marilyn munroe had a threesome relationship with the 'gemini twins' for a number of years. sure, she wasn't the most psychologically healthy, but it seemed to work for her.

then there is the idea that sex can be more recreational than relationship focused so recreational threesomes don't really come to bear on relationship twosomes. unless... there are problems with trust etc in the relationship twosome, of course.

i've known a few people who have had 'open marriages'. i don't think they did threesomes, but each partner was open to sleep with other people. the constraints included honesty with ones marriage partner in terms of informing them. also practicing safe sex. that worked out well for them for a number of years.

i guess there is a temptation to think that the way things are typically done in our culture is the only way or the best way... practices diverge across societies, however, and clue us into the possibility that there can be many different ways of being that are all feasible.