Question: how many possible combinations of 3 letters?
Answer: Since letters can be in any combination or permutation as long as they arent words as your question suggests, any letter can occur in any of the 3 slots thus the answer is simply number of letters in the alphabet to the power of 3, also commonly known as “cubed”. Thus, 26 x 26 x 26 or 26^3 which is 17576.