Release of an N-terminal tripeptide from a polypeptide. The limitation of proteolytic products to tripeptides is achieved by tailoring the size of the substrate-binding cleft: the two negatively charged residues Glu-367 and Glu-393 that are blocking position P4 limit the number of residues that can be accommodated in the binding cleft and thus create a molecular ruler. At the same time, they orient substrates so that the tripeptides are removed exclusively from the N-terminus (By similarity). Belongs to the peptidase S8 family.