In my (entirely humble) experience there's too much pressure over this sort of thing. It is a very society given fantasy. In the interest of fairness I did all sorts of things in my early twenties and a girl I went out with for a year was very into girls. Mostly it was something that she just did, rather than anything to do with I per se. So yes, I've been involved in a couple of threesomes, many years ago, but never arranged or planned. Glad I did it? Well, yes - of course. Now? No. That was then as it were and whilst I was a scamp back in the early nineties I'm more than content with my wife now, and we've been together twelve years, have kids and so forth. I think the fact that both of us were tarts in our youth means that we don't have any hang ups in our middle years.
I'd give more lurid details, but I'd have to do it in the sex stories section to give it that bit of disconnection. Fond memories, aye?
That being said, do whatever each is happy to do. Don't force anything. Don't shy away from anything that just happens as long as you're both into the idea. Don't fret it.