Ninja Gaiden Cheats

You first need to unlock Ninja Gaiden 2. Go to the peristyle passage in the Aquaducts, then bird flip all the way to the top. You'll see a chest containing Ninja Gaiden 3.

Highlight "New Game," then enter the code below:
Left Trigger + Right Trigger and press A button

Beat the game once on Normal or Hard.

When fighting these ninjas, it works best when you block and use a
counter attack. The Y button one works best in my opinion. You may still
be hit however but not usually.

 
Level Password
1 B, Y, R, L, X, A
2 A, R, R, Y, A, L
3 L, B, R, Y, L, Y
4 R, R, B, A, Y, X
5 Y, A, B, X, R, X
6 B, X, X, A, X, Y
7 X, R, X, Y, Y, L

Alternate costume:
Successfully complete the game under the normal difficult setting. Then, hold L
while selecting the "New Game" option at the opening menu.
Plasma Saber:
Successfully complete the game under the normal difficult setting. A new sword
called the Plasma Saber will now be unlocked.
Extra hard difficulty:
Successfully complete the game under the normal or hard difficulty settings to
unlock the very hard difficulty setting.
Unlock all FMV sequences:
Successfully complete the game under the normal or hard difficulty settings to
unlock all FMV sequences.
Skip some FMV sequences:
Press Start, A, Start to advance past an FMV sequence.