Although most of our encounters have been one-on-one with couples, we've had a few threesomes with another man and to a lesser degree, a woman. None of them were planned as such, but usually the result of either not being able to find an interested couple that night at a club or when one pair of the couple was not participating for whatever reason.
In a few instances it was also due to playing with multiple couples and the natural, sometimes uneven, pairing off; which might result in two guys servicing my wife or the one time I had two other women orally sharing me.
So for us, threesomes were more a fall back and the occasional dessert more than the main course of our swinging activities.