The least three-digit number that is divisible by both 5 and 9 is 135.
987
144 is divisible by 12 and 9.
180
900
225
900
999
100. The highest 3 digit number divisible by 9 is 999 (111 times). The highest 2 digit number divisible by 9 is 99 (11 times). The difference is 100.
3, 6, 9
135
531 is one of them and that any 3 digit number whose digital sum is 9 is also divisible by 9