Call of Duty 3 Codes -

Unlock Levels
Hold button in the Chapter screen and enter x2, x2, x2 to unlock all levels.

Unlock Veteran Interviews
Gordon Boulton - Complete the game once.
Sgt. John D. "BUD" Hawk - Complete the game once.
Sgt. Stan Markut - Complete the game once.

Fuel Plant Map Tips
For super easy kills on the fuel plant map, just get up on to the balcony overlooking everything and shoot the opposition, also from near where the trains are, there's a re-spawn point then there will be a crate of oil tanks, crouch or phrone on the right side of the barrels and pick everyone coming down the central column.

Another tip for both modes is when you have a long range gun: KAR 98K, Lee enfield. FG42 etc use it to your advantage by going for people that cant see you or are on the pause menu.

Also on the fuel plant map there are some stacked barrels there are little spaces in between shoot through their because for the opponent to get you they've got to be good at the game to aim 100 yard through a small hole that the bullet will only just fit through.

Last but not the least -- when in multi-player and you're either anti-armour or support you can dish out ammo to yourself.