Hi JimBoy. Yeah I too have struggled with those intense feelings. I am part of a triad, but my wife and GF are very much in the throes of NRE. we enjoy the threesome experience, but they also want time alone. The GF wants time with me and maybe some sexual stuff...but is not quite comfortable with me yet. She is very comfortable with my wife (we are all women) and very much wants to have her spend the night. I have watched them make love and it is very intense. Mostly I think it is beautiful. Sometimes, I feel overwhelmed,a little jealous, and insecure. I feel like an outsider. We interact very well together sexually. I know its my shit that needs to be worked out. My best offer of help is to talk...A LOT...with both your partners. Talking about what you are feeling helps, and you can make a gameplan for when the two of them spend the night together. I myself am working up to that. I want them to have their time and explore their relationship. But it's not easy knowing that you get to spend the night alone. Make sure things are solid with all three of you before you move forward.
One thing I realized is that I am not getting what I need out of our existing poly relationship. My partners are aware that I am looking for something more and understand. You may be in the same situation.