I was in a similar situation with a straight couple off and on for a little over a year. Sexually it was incredible, even liberating in some ways, as there were no boundries in our search for pleasure. I'll admit there was some apprehension on my part the first time, but I look at any MMF situation as one where it's about her (it's certainly more of a female fantasy than a straight males) so once I was in that perspective I could ignore the other wang and act accordingly.
Then again I wasn't competing against ole' White Snake either, but I digress... LOL
The games were endless. Sometimes everything would be planned with all three of us getting together. Othertimes I would get a surprise text telling me exactly what to do to her or make her do to me (or she would get a text). No matter how it came about I would get so turned on knowing they would talk about it (what they want to do, or what they did do) during sex it made me always strive to be at the top of my game to get them as hot for eachother as possible whether all three of us were hooking up later that evening or not.
Would I do it again? With the right couple I would but to say communication is key is the understatement of the year...
Some people aren't as secure with their sexuality and their relationship as they think they are which can lead to resentment, jealousy, and just plain drama if you're not careful in these type of situations and close to both/one of the couple.