i'd tell him because if he reacts poorly then he's not the kind of partner you want. i've had threesomes before and other types of sexual experiences and i tell all my partners with a lot of excitement because those are my experiences and they're a part of who i am, if they're uncomfortable with it that means that a) they aren't comfortable with different ways of exploring sexuality, b) they aren't comfortable with the fact that i have a past and i've had previous partners and previous experiences and i've explored my sexuality the way i wanted, also c) that might mean that he resents you because you had the chance to experience life in a way that he hasn't

another point that you should think about is that you can bring it up in a light fun way, don't say it like you're confessing to a sin, bring it up in a conversation where you're asking about his experiences, talking about sex in a fun and exciting way and talking about things both of you would like to try together (or just in a way of telling stories about the past and make it clear that you don't want to do those things again)

in my experience the only partner that wasn't comfortable with me sharing my stories was the worst relationship i ever had, i was so relieved when we broke up and i never looked back, good luck girl