Jealousy is a feeling; feelings are real.
I don't know if jealousy can ever be avoided all the way in poly or monogamous relationships.
For me, the important thing is to own it. To not place the blame on your partner(s) for how you feel.
As in "I am feeling jealous" rather than "you are making me jealous".
No one is making you do or feel anything. If you are feeling jealous, you have needs that aren't being met, and communication that needs to happen that hasn't yet. Try to think clearly about the specifics.
Is he really making you jealous by spending his evening with her? Or is it just that you were really hoping he'd be around to eat dinner with you tonight because you cooked something special.
Address the specific need. He hasn't done anything wrong by going to the movies, and you need someone to eat dinner with. You probably have a friend who'd come over and eat dinner tonight.
If I try to make sure my specific needs are met, I find jealousy rears its head much more rarely.