How to ease partner into threesome?(serious response only)?

When me and my boyfriend first started dating, he was somewhat interested in having an extra partner or 2 during our sexy time. I was uncomfortable at first because I wasn’t sure during that time if he was only with me for lust or for love so we didn’t do it because he was also kind of unsure on if it would be a good idea.

a couple of years have passed and now I’m ready for a threesome and he’s not. I tried to bring it up but he thinks it’ll cause jealousy among other things like finding a partner. What do you think I should do? Because I brought it up again a couple of times before and he said he hates the idea of anyone else touching me. Should I just completely throw the idea out?
How to ease partner into threesome?(serious response only)?
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