How do you get to the third gym in Pokemon Sapphire?

Answer 1

Simple

step 1:have the 2nd badge in dewford town

step 2:tell to the guy near a blueprint inside Stern's shipyard that you will bring the Devon goods to Stern

step 3:deliver the Devon goods to Capt. Stern

step 4:defeat the 2 magma/aqua grunts

step 5:walk to route 110 and battle rival

walk,walk,walkuntil you reached Mauville city

P.S.:to gym inside the gym battle wally first

he uses ralts so use a dark type