I've never had a threesome, but I have had a foursome lol. A very casual acquaintance of mine asked if wanted to have a threesome with him and a ts girl. I wasn't interested in the ts at all, but said yes just because I saw it as my only opportunity to play with him (he has one of the top 3 biggest dicks I've seen in person; over 9", we measured it).
Turns out, the third person wasn't a ts at all but a reasonably attractive Filipino guy. Me and the Filipino guy take turns giving my acquaintance oral and rimming, but he has his head stuck in his phone. He starts f***ing the living hell out of the Filipino guy, we hear a knock on the door, he tells me to get it, I do (butt naked), and turns out it's yet another guy he has invited over for some fun.
I'll leave it at that. It was definitely my hottest sexual encounter.
But in sum, I would say a forum or app would be a good way to test the waters and be explicit and upfront about what you guys are looking for, assuming there aren't any venues that might cater to a sexually liberal crowd in your area. It saves everybody time and doesn't involve any upfront awkwardness.