Have you read
The Ethical Slut?
Probably the best book on poly ever. It has a lot of information concerning situations exactly like this and how to deal with them. It especially addresses jealousy and how to deal with that emotion. BTW I'm in a MFM V right now. I'm one of the M's and though I have many reservations about the situation. My partner is very open honest and does her level best to love me fully and completely. She is awesome. Him I don't like so much. Such is Karma, ne? Good luck.