I've never done a threesome, but I think porno makes people think they would be cool. Seems like a big hassle to me. Color me uninterested.
They
are overrated... 90% of the time.
The mechanics of 3 bodies just makes it hard, for one thing, but the level of communication (verbal and nonverbal) between 2 people is hard enough; with 3 people it's REALLY hard, especially since it's almost always 2 people who have sex a lot and 1 person who is "new to the group", as it were.
On the other hand... when you can actually get 2 other people who have great chemistry with each other and with you... you may find yourself not wanting to stop until you all pass out from exhaustion. It's worth all the hassle. Really.
And no, I haven't since I've been married, nor do I intend to. Like taking large quantities of LSD and mushrooms at the same time, this counts among the wonderful experiences I had... when I was rather younger than I am now.