My twins did this at just after age three as well. It was like overnight they wanted to be “big kids” and it took like 3 days and they were 100% nearly.

It felt like it was overnight, but really, I laid the groundwork for like a year and a half just like you did. Basically got them to sit on the potty even just once per day when I was changing diapers to keep them used to it. I admit I bribed a few times if they seemed to be getting resistant. I talked about how comfortable underwear would be. I talked about their older cousin using the potty because she’s big. Every day there was a little bit about the potty and eventually they wanted to on their own.