Which three functional groups are found in the general structure of amino acids?
Amine, alkyl, and carboxylate.

I'd guess the part that you're having trouble on is that the alpha carbon in the center of the generic AA structure is part of an alkyl functional group
Edit
The original answer that I gave might have been too clever for its own good. Depending on how one looks at the question, the third functional group could instead be the functional group of the particular side chain, i.e. for Alanine it would be a methyl group, for Tryptophan it would be an indole group, etc.