Sentence 1
I get up very early at 4:30 am.
Subject: I ( First person singular)
Main verb: get up which is first person singular.
The subject and verb agree.
Sentence 2
My sister doesn't get up early.
Subject: My sister ( first person singular)
Main verb: get up( first person singular)
Sentence 3
She often gets up at 9:00 am.
Subject; She ( third person singular feminine)
Main verb: gets up ( third person singular)