We have these two clauses:

(1)  my hundreds digit is 2 more than my ones digit. 
(2)  my tens digit is 1 less than my hundreds digit.

The words "hundreds digit" is the subject in (1), so rewrite (2) such
that "hundreds digit" is the subject in it also, using "more" instead
of "less":

(1)  my hundreds digit is 2 more than my ones digit. 
(2)  my hundreds digit is 1 more than my tens digit.

Then write it as one sentence, starting with (2) and we have:

"My hundreds digit is 1 more than my tens digit and 2 more than my ones digit." 

That makes it easy to write all the possibilities:

210, 321, 432, 543, 654, 765, 876, and 987.

Edwin