As usually guys are the ones who are turned on by the thought of a threesome, it is very rare to find a single girl. They are afraid that they would not only be used by one, but two people, and if they join their best friend and her partner then their friendship would start dying soon, since every time they meet would remind themselves of that threesome.
While a guy would never think of joining his pal and his partner ever, if they are single, they have nothing to fear for if they join a couple, especially if they make it clear that they are not into bi actions. What they can expect is to have sex a strange woman while her partner is watching, if this is all that the couple wants to try, or they might even be satisfied if you just watch them. If they are lucky, however, then they can experience what a double blowjob, an anal penetration or a double penetration is like. It always depends on the couple and on how easily they get on well with them. There is no need to feel tense, since couples are likely to help them to feel relaxed, as well. A glass of wine is probably sipped slowly before they lose their clothes, too, because this is the easiest way to find the common voice and to get loose for something hardcore.
So, if it has been on your mind for a while now, then try what it would be like with this couple!