I've been "taking it slow" with someone, and just realized we might not have the same communication expectations

So I've had a crush for a while, we're both poly and aware of each others feelings and are taking it slow not official yet. My issue is that they got a partner a week ago and didn't tell me about it. I know I'm probably not entitled to know this, but is it bad that I'm upset about it? I mean we can have other partners but I thought it was like, idk, just a good thing to bring it up? Like they know I've been in a long term relationship for almost three years.

This is a perfect opportunity to have a conversation with your crush about expectations and clarity! It sounds like you assumed that you two were intimate enough that it was reasonable to expect that they would bring up something like dating a new person - but you got evidence that they’re not operating on the same set of assumptions. Neither of you did anything wrong, you’re just in an ambiguous situation and that’s caused a mismatch of expectations.

But now you have something to bring up to them to actively reduce that ambiguity and clarify those expectations. You can say something like “I’m so happy for you to be in a new relationship - but I have to admit I was kind of surprised to hear about it, because you never mentioned that you were heading in that direction with someone else! It’s not that I think you’re obligated to tell me everything, and I’m not angry - but we’ve been talking about our feelings for each other, and even though we’re “taking it slow,” I thought we were in a place of working out what a relationship might look like - and I guess I just assumed that conversations about other relationships would be part of that! I’m realizing now that I can’t just assume we’re on the same page, so I’d like to talk to you about your expectations, and how you define this thing we’re doing together.”

It’s not an accusation; you’re not saying they did anything wrong or asking for an apology. You’re just realizing there’s something that has gone un-addressed and probably should be addressed! It’s like, if I’m going on a trip, I always pack a bathing suit because I figure most hotels have pools, and if there is an opportunity to go swimming, I want to be able to swim! But say I’m planning travel with a friend, and I don’t think to tell them “Oh yeah, I try to include swimming in any vacation if possible.” And they come over and see me packing for the trip, and they see a swimsuit, and they say “Hold on, do I need a swimsuit for this trip? Are we only considering hotels with pools? Are we trying to include a beach day in the itinerary? Should I also bring a towel and sunscreen?” These are all valid questions and probably something to discuss before a trip - but it’s not a huge moral failing that I didn’t bring it up; they just noticed my swimsuit and it occurred to us to clarify what page everyone is on regarding this aspect of the trip.