Thoughts on a threesome

So my husband and I are considering having a threesome. FFM. We have been together for 10 years and this is just a way to spice things up and keep it exciting. We have toys and everything so been there done that. Just wanting to hear other people’s experiences. What were your ground rules? Did you know the third party? How do you keep one person from feeling left out? All thoughts will be considered. Thanks 😊