Several years ago I asked my husband if we could add a man in, and he readily agreed. Then we tried FMF as well. Now threesomes are our favorite extra-curricular activity. Nothing brings you into the present and out of your well established lanes like having someone else in your bed. Too many people means less good connection, so three is the perfect number for us, but you are absolutely right that you have to have the correct foundation in your relationship for it to really be enjoyable.