I've had the good fortune of a few threesomes, but this was my first. It happened back in my university days.
I was about 23 at the time. I was out at the bar with two of my buddies. Had a good night, drank far too much, and were headed home. We took the magic bus (a free booze bus), which dropped us off down the street from my place.
As we walked down to my apartment, a cab pulled into a driveways right as we passed. Two young girls got out of the cab, clearly drunk. I called out "hey," and they said hi back.