Axis and Allies 1940 Multinational Defense, AAA and Radar
It depends on the exact wording of the rule that "radar increases the hit score". If there is nothing beyond those words, then it would logically apply to the unit; in your example the Allies have one British AAA that can take up to three radar-assisted shots, leaving the American unit to take the other two without the bonus. (It would be logically possible to decide to take two radar-assisted shots and three 'manual', but this is unlikely in a real game.)