I would like to add, you actually can make the two ideas K and I had work. Mine is a little easier. See, here's the thing. First, you talk with your girlfriend up front about a fun little fantasy you want to play out, and if she agrees, you stop and pick up a pizza on your way home from work. Then . . . have fun with it.
K's idea is a little harder, because now you need a girlfriend who's okay with a threesome, but again, talk with her about it first, and if she's cool with it, and one of her friends is cool with it . . . have fun with it.
Happening "naturally" on its own, like in porn movies? Ah, nope. Remember, even porn movies are scripted. Even if most of the time the script writing is horrible.