Not quite, since order isn't important. If it was, then there would be 1000 permutations and you would be correct. But rolling 4, 5, 6 for 15 damage is the same as rolling 6, 5, 4 or 5, 4, 6 or 4, 6, 5, etc.
Since order isn't important, there are 220 possible combinations, with some totals more...