Responsible for the incorporation of O-acetyl groups into the enterobacterial common antigen (ECA) trisaccharide repeat units. Catalyzes the acetylation of both cyclic ECA (ECA(CYC)) and phosphoglyceride-linked ECA (ECA(PG)). Bacterial outer membrane biogenesis; enterobacterial common antigen biosynthesis. Mutant is defective in the incorporation of O-acetyl groups into both ECA(CYC) and ECA(PG). Belongs to the acyltransferase 3 family.