The $3B from USA is specifically military assistance, of which 74% is to be spent for hardware / services from US contractors. The aid provides equipment, Israel provides the soldiers.
Economic aid from USA has been phased out completely.
Historically, a large amount of the military aid was provided to help make up for the loss of Sinai in the 1979 peace treaty with Egypt. Promises of aid from USA to both Egypt and Israel over many years was an important part of the 1979 peace treaty.