There is a massive document on everything you need to migrate from SDL2. It covers literally every symbol that changed from SDL2 to SDL3.
https://wiki.libsdl.org/SDL3/README/migration
And I plan to do some YouTube videos to show how to migrate your game. It took me about an hour of poking around to get ioquake3 ported last night, I'm going to do it from scratch tomorrow and record it, explaining the details as I go. Stay tuned. :)