It’s not a permutation, either, at least as defined by mathematicians.
I’m not sure if there’s a commonly accepted term for it, but it’s the result of a Cartesian product {0-9,A-Z} X {0-9,A-Z} X {0-9,A-Z}. And it’s related to the counting principle.
If each character was only used once, then, yes, it would be a mathematical permutation of 36 alphanumeric characters taken 3 at a time.
The formula for that would be:
P(n,r) = n! / (n-r)!
In this case, the 36! / (36 - 3)! = 36!/33! that was mentioned earlier.
I think it’s a “combinations” rather than a “permutations” problem. But yes, it’s with repetitions (more commonly called “replacement”): Combinations with replacement.
The logic of using the word “replacement” comes from the scenario where you are drawing various colored marbles from an urn (in elementary stat and probability, it’s always an “urn”), replacing each marble before the next draw, so that the same marble could be chosen again.
ETA: I partially take that back. It’s a permutations problem.
“With replacement” means the same letter or digit can appear more than once, e.g., ATA.
“Permutations” means the same letters in a different order count as a different result: ATZ is distinctly different from TZA or ZAT, and these would all count as separate possibilities.
“Combinations” means that the same letters in differing orders would all be counted as the same result: ATZ, TZA, ZAT would all count as just one result.