How many 3 letter combinations?

17,576 possible combinations

* * * * *

Not quite! This is the total number of permutations of 3 letters out of 26. AAB is counted as being different from ABA or BAA. But these are the same combinations.

Thus there are 26 combinations of just one letter.

26*25 combinations of two letters and

26*25*24/(3*2*1) of three.

In all, that makes 26 + 650 + 2600 = 3276