I think 3somes is the one aspect of sexuality that is truely homosexualy friendly. The easiest way to get all parties involved interested and to enjoy it, is to ensure that theyre all going to like the 3rd member.
With my partner and I, we have a couple of rules when thinking about it.
First and most important rule is, to draw a line between it just being sex and it actually meaning something. When having sex with my partner its all about the love and enjoying things because theyre being done with him. When we invite another guy into the bedroom, its all about the sex. This prevents the jealousy side of things from festering after the event.
Second rule: Pick someone you both find attractive and would genuinely want to take home with you had you met them as a single person in the pub. After a few years of experimenting with 3somes, we've found it helps to meet them first in a neutral location to ensure that you find them both physically and personally attractive. Sex for us helps if you can have a good conversation first then take them home and bind them to the bed...
Third rule: make sure you're sharing the attention, it can be all too easy to spend more time with the new guy than with your life partner. Theres always the temptation to explore things (and indeed I believe 3somes can be a welcome release to a long term relationship - allowing you to experiment with other things in order to learn new things that can be put to practice with your partner)
Fourth Rule: Never do it with someone that only finds one of you attractive or obviously fancies one of you more than the other, ignoring this rule quickly leads to jealousy.
Fifth Rule: Never do it if your relationship is already on rocky ground. You need to be secure in yourself, how you feel about your partner and how they feel about you. It is all too common in the gay community for a 3some to end up with a broken relationship and the other 2 getting together instead. This obviously not being the aim of 3some.