Philadelphus inodorus grows along the Wissahickon trail just a bit downstream from the entrance to Forbidden Drive.
It has 3 flowers per cyme (and some single flowers), hairs in the underside leaf vein axils, and it is arching (and definitely not upright).
Taylor’s Flora of the Vicinity of New York (1915), which includes Philadelphia, has Philadelphus inodorus as not native to Philadelphia:
And Porter’s Flora of Pennsylvania (1903) has it as naturalized in Pennsylvania:
In the Flora of Bucks County (Benner; 1932), P. inodorus is noted as “Escaped from cultivation and established along steep bank, New Hope”; and in the Flora of Lancaster County (Small and Carter; 1913), P. inodorus is noted as “Nat[ive] of the s. U. S.”.
It is not in Barton (1818):
In the New Britton and Brown Illustrated Flora (Gleason; 1952), P. inodorus habitat and range are noted as: “Stream banks and moist hillsides or cliffs, chiefly in the mountains, e. Pa. ; N. C and Tenn. to Ga. and Ala. ; frequently cultivated and occasionally escaped northward.”