This isn't a play rehearsal. While you can plan out certain elements of a threesome, there's no script and there are certainly no guarantees. Having too many expectations and demands will just only get in the way of your enjoyment.

Accept that you have minimal control and know that the most you can do is be open-minded, attentive, and willing. The rest will happen as it happens. Let it. Even if it's the worst sex you ever have or you get completely shut out (it happens), you'll still have a killer story for dinner parties when you're old and boring.