Conditional validation using validators?

What about conditional validation?

For example, I have a form with list containing three items and three text fields. Basing on what item user selects in the list, he is obligated to fill-in text field no 1, 2 or 3 and may leave up others empty.

Can it be done using build-in or custom validators? Or the only way to achieve this is to write own custom validation rule, attach it to all three text fields, and check if condition is satisfied using some ifs inside?