Threesome Advice

I haven't posted on Reddit much, mostly lurk, but wanted to get some advice. My fiance and I are discussing having a threesome. Both of us are fully on board for it and excited. But I wanted to post on here and get any advice from those who done it before. Is there anything we should discuss beforehand? How can we make sure this doesn't affect our relationship? What's the best way of looking for a third? Thank you in advance!