Fifteen bacterial colonies growing on a complete medium are transferred to a minimal medium. Twelve of the colonies grow on minimal medium.

The serine biosynthetic pathway is a three-step pathway in which each step is catalyzed by the enzyme product of a different gene, identified as enzymes A, B, and C in the diagram below.

Mutant 1 grows only on  In addition to growth on , mutant 2 also grows on  and . Mutant 3 grows on  and . Identify the step of the serine biosynthesis pathway at which each mutant is defective.