Hi, welcome to the board.
Like many other married couples (myself included) you got started in polyamory seeking a "unicorn" to "add" as a "third" to your "couple," to have fun 3way sexy times... maybe both you and your h think 3ways are hot. I know it is the most common fantasy men, at least, have.
Many couples think only having 3way sex (and usually, even only 3way dates with the unicorn as well) prevents jealousy. Also, many couples require that their "shared gf" not date anyone else, again, so the couple won't be jealous.
All this leaves out the feelings of the "unicorn" completely. Maybe she desires one on one sex with your h, and with you! Maybe she likes you 2 a lot, but wants one on one sex with SOMEONE, and if she can't have it with you or your h, she will seek it elsewhere.
Jealousy is not something you avoid by limiting the behavior of another adult. Jealousy is based on fear of loss. If you're going to be poly, you, personally, your h, personally, need to feel the jealousy, investigate your fear of loss, your envy, your low self esteem, and come out as more self confident, openly loving people who do not cling to the fear, but feel it, deal with it, and move beyond it. Otherwise, you are not poly, you're just fucking around, and everyone is gonna be hurt.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37