American shopping malls of the mid-to-late 20th century were a decent bet, but I think they fail because a) they're very pedestrian hostile, b) they tend to be owned by a singular landlord that can enforce policies that discourage gathering, and c) they just kind of smack of artificiality. But bazaars, street markets, and other commercial areas have certainly been third-places since before long before it was a defined term. Heck, the ancient Greeks were quite proud of their agora.