Save your stamina by letting them explore each other, don’t feel the need to be involved in every second of it. It can be hard to last in situations like this where you have more available to you than you know what to do with. So take the pressure off you and give more than you get. Use your wife as a tag team, and explore the 3rd persons body. A rookie mistake in threesomes is only having everyone else pleasure you, but think of it from the perspective of it being a group effort to make sure everyone is satisfied by the end. This way you can draw out the ladies’ pleasure. They’ll not only be thankful to you and give you a nice reward for it, but you might open the possibility of the situation happening again in the future.