well, the second digit must be 9 more than the third. as in 09...or, it is the complement to 9 (as in 1), as in X23,so the switched is X32 (nine more).
Complements: N,N+1 (as in 12, 23, 32, 43, and so on.
Not the second hint..the first and second:
BAx-ABx=90 so a,b must be complements also, as above.
Lets check; 123
first hint: 132-123=9
second hint:213-123=90 checks
Lets check another: 456
first hint: 465-456=9
second hint:546-456=90 checks.
so there are a multitude of answers for what the number is.
HOWEVER, the answer to the question asked is interesting.
In the first case, 321-123=198
in the second case,654-456=198
hmmmm, very interesting. Why don't you try a few more?