Keep in mind, jealousy issues are NOT about you, they are about the insecurities of your partner(s). If they trust enough in the strength of your relationship, then they will get past most of these feelings naturally.
Most of the better books on polyamory have a chapter devoted to jealousy issues. I'd suggest latching on to one and share it with your jealous partner - take turns reading aloud and discussing.
Ciao ~ Mike