There are 10 letters in the set {a, b, c, d, e, f, g, h, i, j} which is the pool of letters to choose from when making these three letter codes.
We have 10 choices for slot 1
Then 9 choices for slot 2. This is because we can't reuse the choice for slot 1
Then 8 choices for slot 3
Overall, there are 10*9*8 = 90*8 = 720 different permutations
Answer: 720
Note: you can use the nPr permutation formula with n = 10 and r = 3 to get the same answer