I've done threesomes 5 times with my wife, my best friend and our ex boss!
The first time was soooo exciting to me. We did it in a hot tub, with my best friend. It was a strange feeling afterwards. I felt kinda guilty, like I had forced my wife to do it, so I apologized to her. She said it was OK, but had enough of that thrill. 2 hours later, she wanted to go back for more!!!
It's awesome to do, and I don't mind sharing my wife, just don't make a tape of it!!! That was the only blunder I made, and it caused friction for awhile, between my friend and I.
The last threesome my wife and I did was back around 2000 or 2001? To this day, we fantasize during sex at times about doing a threesome. The sex is hotter now than ever before. You just have to trust your partner, that they won't go do side things and everything is kosher.
So go ahead and do it!!
You only live once, and life is all about having fun in my opinion.