There are 10C3 = 10*9*8/(3*2*1) = 120 combinations.
There are twelve possible solutions using the rule you stated.
98765432
9876543
It is 98765.
987654
There are twelve possible solutions using the rule you stated.
There are 840 4-digit combinations without repeating any digit in the combinations.
98765432
It is 98765.
9876543
123,456,789
9876543
987
987654
I believe it is: 987,654
987654321
10234567