The children are 2, 3 and 5.
If I was AB years of age (which is really 10 x A + B), my youngest child would be A years old, another would be B years old and the third would be A + B.
The total is therefore:
(10 x A + B) + A + B + (A + B) = 33
10A + B + A + B + A + B = 33
12A + 3B = 33
The first value for A that makes sense is A = 2, making B = 3. Although mathematically I could currently be 17, which would make my children 1, 7 and 8. This would have made me 9 years of age when I had my first child, which I wasn't! QED.