There are lots of ways as to how it could be set up, I had one about five years ago with two girls that were in my social group when I was living abroad. We got drunk, spontaneously went skinny dipping on the way home and then back to mine, just used lots of "you only live once", "you don't want to live your life with regrets you were to scared to try new things" and similar framed things, didn't even talk about a threesome, just kissing one girl and then started kissing the other an happened. This worked well as the girls we're obviously experience seekers, as they were living abroad at the time and also knew that people from home couldn't judge them for it.

The threesome itself wasn't amazing, I didn't really fancy one of the girls at all, we were all really drunk and they were a bit hesitant, I ended up finishing by concentrating on the fitter one and practically ignoring the other one.

Is all you really need is to get a fuck buddy who wants one or you can convince to have one, once you have this, you've won half the battle and can go out to pull another girl with her or just need to have both of you on the lookout for a third member.

Trying to get one in a ONS kinda way is difficult and but getting two girls back to your house is definitely doable, in fact sometimes girls would rather their friend comes back as well, if you have a reasonable extraction method, like after party at ours or whatever, then it's actually making it happen, not very easy IMO unless you strike lucky and find a couple of slags.