How many possible combinations are there with 3 keys and 4 keystrokes?

Answer

If you have to choose four times from three possibilities (A, B and C) you have 81 possible combinations. You can choose from three for the first attack. Each of those three can then be expanded in three ways at the second attack. That gives 9 combinations (to write them down: AA, AB, AC, BA, BB, BC, CA, CB CC)

Each of those 9 can be expanded in three ways at the third attack, so that has 9 x 3 = 27 combinations of three attacks

Finally, you can expand each of those 27 a fourth time three ways to get 81

On the attached document you see the 27 combinations that start with A.