Brother: blood related (same mother and father)
Bro: basically means friend, like "Hey bro did you see the black widow movie?"
Bruh: usually if someone does something dumb, but can also be used instead of bro. Say a friend does something dumb, you could say "bruh". Or you could say "bruh have you seen the new black widow movie?"
Hope that clears things up.
-cuberdude
