Crusader Kings 3 isn't an RTS game. If you're expecting something like Age of Empires or Command & Conquer then you're not going to get anything close to what you expect.
I'd say try Crusader Kings 2 first. It's free to play now and IMO better than CK3 still in every department other than graphics. CK3 is still missing a bunch of stuff that CK2 had.