My girlfriend and I have trouble as well. It's hard to find someone trustworthy to do it with. You can always find swingers. It's anonymous. You never have to meet them again. Then again you don't know where they've been, if they're crazy or not, if they'll take things too far, etc. So then you go "okay. I'll find a friend or someone. That'll be safer and better." But then what if someone gets jealous? What if it's weird afterwards? And so on, and so on. I'm sure it's silly to be so worried because it'll probably turn out fine but I guess it's good to be picky. It's so much harder because of that 3rd person. You want everyone to enjoy it and feel good but there's a ton of factors that go into play as well. But once you find that good 3rd person, it'll be worth it.
The problem with 3somes is finding partners that are equally into one another. If both girls don't want to fuck each other as much as they want to fuck you, then it makes for a rather dull 3 some.
If you want an MFM threesome, on the other hand, you'll have a line-up down the block of willing potential participants - and I'll be at the head of the line!