How to get over jealousy in a semi- open marraige?

So my husband and I both really love woman so occasionally we have 3somes. Hes an amazing husband, has no desire to do anything with other woman without me ( hes shown me that time and time again) but still i get jealous over stupid little things and make him wrong for it..when really its jus small petty stuff. I love that this is a part of our life and we are very much passionatly in love. I jus wish i could get over some of that jealousy and let him be free to be trusted in the situations.. and out of them. We recently had a 3some w a mutual friend so they text eachother occasionally.. even flirt a little bit (hardly tho) and that should be ok right? i talk to her all the time and im very flirtatious..so why do i get so upset when he does the same.. would it be diff if it was man-woman..woman-man? or is it the same even tho its 2 woman and 1 man...some advice please :)