Was is planned or spontaneous?
Aside from the obvious sexual gratification*, did it have any effects - good or bad - down the line?
How did you ensure that your partner would be completely comfortable with the experience?
(*10 years of porn have conditioned me to believe that a threesome is the Holy Grail of human sexuality and that after I have done it, I will bask in a state of ultimate bliss forevermore, safe in the knowledge that my life is finally complete. Obviously that isn't the case, but it's hard to argue with your penis. So... input welcome. Preferably based on 1st-hand experience.)
(PS I'm referring to a FFM threesome. A MMF threesome is out of the question. Hypocrisy FTW!
That being said, if your past contains MMF, MMM or FFF cavortings, then feel free to pitch in as well.)