Cleaves the N-terminal amino acid of tripeptides. Has a broad specificity for tripeptides with no clear preference for a particular tripeptide. Tripeptides with proline in the second position are an exception and are not hydrolyzed. Does not hydrolyze dipeptides, tetrapeptides, or oligopeptides (By similarity). Release of the N-terminal residue from a tripeptide. Binds 2 Zn(2+) ions per subunit. Homodimer. Belongs to the peptidase M20B family.