Serine proteinase. Releases tripeptides from the free amino terminus of proteins. Has a requirement for Pro in the P1 position, but is inactivated by Pro in the P1' position (By similarity). Hydrolysis of Xaa-Xaa-Pro-|-Yaa- releasing the N-terminal tripeptide of a peptide with Pro as the third residue (position P1) and where Yaa is not proline. Belongs to the peptidase S9B family.