As you cannot repeat the numbers it is a permutation (not a combination) for the sake of calculating the number of possible outcomes.
The total number is 10! / (10 - 3)!, in English Ten Factorial divided by the Factorial of ten (number of available digits) minus three (number of times the digit is used).
I get the advanced math, but this was in context of how may different numbers would need to be tried to open the panniers...
If the OP only tried the 720, there is a 28% chance he would not open the lock as it could be one of the 280 sequences possible that were not attempted. It's all clever semantics on the word 'combination' rather than on the actual reality of opening the pannier's lock... I get that.. but even so...
In reality, the numbers can be shifted off center in tiny increments and these count too... so the number goes up by 'a lot' (not a mathematical term).
It's a moot point as the pedantic dealer obviously only tried the 720 on the permutation lock before declaring defeat and the locks faulty!