The following are known Action Replay Codes for Quake III Arena on SEGA Dreamcast (Dreamcast). These codes were also known as GameShark Codes and in most cases will work on both devices.

Device Compatibility: Action Replay & GameShark

P1 health single mode only
FD299CC6
00000064

All weapons do 100 damage
43721843
0000719C

Health pick ups double your health (good and bad)
3C0923C0
0000333C

Ammo Booster
AA5B56F0
00007101