Call of Duty 3 - Unlock all levels and bonus content

Night Drop mission: Easy completion:
When you meet up with the French rebels you will have a choice to go left to the Flak 88s or go through the house to the right. Go left to the guns. Kill all the Nazis around the gun and in the ditch leading to the next gun. Then, blow up the 88 and move on to the rest of the guns. After you blow up all the guns you will reach the house that you would have had to take if you went right. When you go in the house you must defeat a Nazi that is standing there waiting for you. Kill all the enemies in the house and fall through the burning floor. You will see you allies there waiting for you.

The Island mission: Best gun early:
On the second mission, "The Island", there will be a man under the second bridge with a FG42 scoped. That gun will get more ammunition every time you kill someone. The gun starts with 96 ammo remaining and you can kill about 150 people with it not including melee. After completing that mission you will have that gun on the third mission, but you must save the gun through the third mission to get it in the fourth mission and so on.

The Mace mission: Scoped FG42:
When you get to the part where you must keep the Nazis from the trenches, do not listen to the men. Go to the right and follow the path. You will find an FG42 scoped leaning on a crate.

Veteran interviews:
Successfully complete the game to unlock interviews with veterans Gordon Boulton, Sgt. John D. "Bud" Hawk, and Sgt. Stan Markut

Move friendlies:
To move your friendlies, use ADS and aim at them, then press Fire. Do not worry as you cannot shoot at a friendly. Your friendly should move from his location. This is useful when you are trying to enter an area blocked by a friendly

Hold Your Breath:
You can hold your breath to steady your aiming and thus hit on target, this is most useful during multiplayer matches when you are using a Sniper Rifle. You can hold your breath by pressing the L3 button.
NOTE: This will only last a few seconds before you have to breath in/out again.