The questions ask for all permutations of length 3. What you've described is the Cartesian product:

    import itertools

    print(len([x for x in itertools.product(range(10), repeat=3)])) #1000
    print(len([x for x in itertools.permutations(range(10), 3)])) #720

itertools.product essentially does a nested for loop.

The reason it's less is because each number has to be different, so you can't have 000 or 111, etc.