In order to max out your health, magic, and item meters during bonus play in God of War 3 for PS3, you’ll need to find Hades’ Helm. Once you kill Hades and jump into the River Styx, swim down and to the right against the current and you’ll find the helm at the very bottom of the river.
Note that this is a bonus item and can only be used during bonus play, which can be enabled once you beat the game at least once. Turning on bonus play automatically disables trophies.