Is he cheating?

So, my boyfriend of three years (we are both in our mid twenties), who I share a purchased house and dog with, has been caught in the act in messaging and meet-ups and says it’s not true or that it’s just friendly. For context, he’s a social guy, and has always had female friends. There were cases in the past where he was too flirty with them and I had to ask him to draw boundaries for example, not send them a lovey song link saying goodnight (he said he didn’t pay attention to the lyrics and just liked the melody), or had a girl he knew for a ong time send him a harmless selfie to which he replied “you’re a gorgeous lady”. This girl, was the same girl that he used to like (and I believe still liked even while dating me) and almost went to a different country for. In fact, he left me at his mom’s house abroad while we there, so he could meet with her alone for the day. I trusted him though and know nothing physical happened.

Anyway, fast forward to now, and he doesn’t speak to her much. However, through his Twitter and Instagram, he follows and messages girls (all Ukrainian as he loves the culture and is from there – he has a type although he claims he doesn’t) – they’re all attractive of course, and he jumps into their DMs by just asking a question like “oh what do you think about the conflict? etc. Then strikes up an e-friendship with them. Again, nothing weird but still makes me wonder.

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Today, I went into his Instagram account… yes I was in the wrong but it’s because I strongly sensed something was up. He told me this weekend that on Sunday he wants to spend it alone… which is sort of weird because the past few weeks he has been going to “Ukrainian meet-ups” at the pub alone (or maybe meet with the girl instead there), whereas before he would invite me to these events. He recently met this same girl at one of them and invited her through Insta to watch a Ukrainian play tomorrow in London (date material) when he claimed that he wanted “alone time”…. it makes sense why he isn’t bringing me. He also made excuses like oh it’s only in Ukrainian, whereas I did digging and found there’s English translations so it isn’t for that reason. I went into his insta today to find out the real reason I’ve been left behind recently… it could be her. I’ve had several cases where I feel he breached my trust. Even after reading the messages, he claimed he’s going alone… I literally read verbatim his texts and he changed toe story to “yeah but we’re meeting as a group there because we discussed watching this play last week at the meetup”… they didn’t discuss it because in the messages the girl asked about the venue, time, and where it is. She didn’t know about it earlier. Then got angry when I apologised for going into his private messages, but that he should understand how I feel as he has massively lied to me. In the DMs to her, he also said “you’re like a skavarotka” in Ukrainian, which translates to “youre like a stove-top” e.g. “hot”. I told him this, and he said he was referring to an artist by this name… skavaortka. Literally comparing her to a Ukrainian scriptwriter by this name. He is now super mad at me though and changed his details etc and won’t come home today. I’m lost as what to do, as we share so many things together and he really was my best friend. This is breaking me and I’m a crying mess. Advice please. I told him I’m hurting and to fix this and instead he hung up and fumed.

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TLDR: Boyfriend is showing signs of cheating. Am I right to think this?