The Harvard Historical Commission voted 4–1 Wednesday night to deny Fivesparks permission to install three modern steel sculptures on the grounds of the former library at 7 Fairbank Street, in the town’s historic district. The decision came after a public hearing that drew dozens of participants and heard competing arguments over both the appropriateness of the artwork and the commission’s authority to regulate. The Sept. 2 hearing was the continuation of a process that began earlier this summer, after Fivesparks sought a certificate of appropriateness for three sculptures donated by the Rappaport Foundation: “Low Boy,” a polished steel sculpture by…