Was there ever a case where it caused a problem in the relationship? Especially after the first time you ventured out there into 3some world?
The first time I did it I was the third. Didn't cause any problems for me, or anyone else that I could tell. The first time I did it in a relationship was with an ex of mine. The guy I was with later told me that he only did it because I wanted to, when in fact I thought it was mutual. I was young then, so we didn't have the communication thing worked out. He and I didn't work out, turned out he was way more jealous and possessive than I thought he was, he was pretty much just going along trying to appease me - which I never asked for nor did I expect, and that never works for me.
The only time there has ever been a serious issue with the threesome itself, was one guy who was romantically interested in my partner and we didn't spot it, he pretty much jumped in to get to my partner. I suppose if I was a jealous person that would have really fucked with me, but then evidently the guy was trying to get my partner alone, and got told no way, the guy then became a huge ass. So we disassociated ourselves from him.
Look sometimes they're great and sometimes not, the rules I have, evolved over time and experience, I suspect that everyone will have their own set. I also suspect that the younger you are the harder it is to just take it for what it is without all the insecurities and issues. In any type of open situation those are killer things. Insecurity leads to jealousy which feeds on itself until it explodes.
A really good threesome for me is all about fun, and everyone needs to be on the same page. I don't mind if the guy is more into my partner than me, so long as we all get to play, It kinda gets me all hot and bothery actually watching some guy lust after my partner, chances are the next time it'll be the other way around. That's the thing though, you have to be capable of recreational sex, sex that isn't a reflection somehow of your self image.
If you think that you couldn't watch your partner fuck another guy, don't do it. If you think that your partner fucking another guy means he's less committed to you, don't do it. If you think that your partner will find the other guy more attractive than you, don't do it. If you think your partner wants one for reasons that are associated with you - i.e. he's not attracted to you anymore (true or not) don't do it. If you have hang ups about monogamy even a little bit, don't do it.
Only go through with it if all of you understand that it's about fun, not relationships or egos, or commitments.