The following are known Action Replay Codes for Mega Man Zero 3 on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
a7646c9a
e30f0172
4a815e78
cb3ea968

Infinite Health
3fe2985a
bee1302b

Infinite Lives
bc0e0332
375f7fd7

Invincibility
2a2059a9
9360b07d
fa24f5ed
b38fd4a8

Super Jump
1e04aa99
a8c931fa
587328c4
9c565c5f

Have All Armor And Weapons
46bea465
9a49a850

Have All EX Skills
d294b609
be9aead3

Always Get S-Rank
4f4593be
9001d638