Gimp 3 is scheduled to be released in May, around the time that Debian 13 is about to come out. Given that Gimp is never on time, and that Debian will only include stable software in their repo, you won’t see Gimp 3.x on Debian for another 2.5 years (the next major release).
However, don’t fret. There’s a way to run Gimp 3, even now, without overwriting the 2.10.x version of Gimp that comes with Debian: https://github.com/ivan-hc/GIMP-appimage/releases That’s how I run gimp 3 on my Debian too, I just download the 3.0-rc1 .appimage file, make it executable, and it’s up and running.
I don’t like flatpaks. Some builds don’t support printing, for example. Same for snaps. That’s why I always prefer appimage from these types of binaries, but my favorite always remains the repo versions.