This is a very complicated subject. Relationships between just two people are very complicated with lots of emotions, feelings, desires, hurts, truimphs and imperfections. Adding a third personality just exponentially increases these feelings. That said, in the past I have been invited to participate in a 3sum that was planned and a few times where it sort of just happened. I have no doubt my "welcome" had a lot to do with the size of my endowement and my over abundant libido. Most of the time it worked out ok. There was the expected feelings of surprise, shock, excitement and sexual pleasure from the folks I was with. But in at least two of the situations, after having had a few 3sums with me, the woman desired in subsequent 3sums to have have some "time alone" with me and that pretty much went over like a lead balloon with the male partner. By the way, in both those situations the husband was large (8-9"plus) but not huge. In one of those situations it got to where she only wanted to have me touch her while he could watch and that was also exciting for him but it also frustrated him. That was understandable and made me uncomfortable as well because of his predicament. In both those two relationships, I decided to end the 3sums but try and still remain friends. Both women took that negatively out of disappointment, and the men took it negatively out of jealousy or hurt. Both marriages fell apart within a year of my ending it with them. Sadly. I am not sure how secure the marriages were before we got together, but I know I did not really "help" and possibly speeded up the demise of the marriages. One woman contacted me immediately upon her divorce being final and was very persistant about pursueing me and the other lady contacted me about the time her divorce was filed! I declined to have sex with either lady afterward though I did try and remain friends and such. Hard to do that without leading them on or being unkind. The 3sums I have had that did seem to work out without jealosy or hurt feelings, and so far as I know, no threat of divorce, was when the other male was very huge themselves (12" plus and more). Not as large as myself but certainly huge and "special". Perhaps the females in such relationships are used to the male egos, or not so captivated because they have someone enormous at home. Perhaps it is a crude comparison, but their relationships may have been stronger because they recognized they had something special at home and I was just the biggest, very novel and exciting dessert. One they never thought they would "find", but not willing to risk the great one they have at home in the pursuit of. That thought might be appropriate for some of the women and men, but for a few, I think they were just thrilled to find someone even bigger than their husband/boyfriend and wanted to see how he would react to watching them with me. Life is complicated...sex even more so. And it is hard to keep up with what is going on in many peoples minds.
So I would recommend that you very carefully consider all the things involved and not just let your libido go in to autopilot and say "yes". You might end up regretting it greatly or it could turn out to be a wonderful experience for everyone.