LilMak,
Couple points.
1) The US Navy didn't say one M2 20mm hit was equivilient to three Browning .50 hits, they said one M2 20mm cannon was equivilent in effective firepower to three Browning .50s.
2) One Hispano 20mm instalation weighs significantly less than a triple Browning .50 instalation.
3) The Hispano and Browning are both abnormally heavy guns compared to other WWII aircraft guns like the German MG151/20 20mm, Japanese Ho-5 20mm and Ho-103 12.7mm and Russian B-20 20mm. You could put in six Ho-5 20mms and only gain a slight amount of weight over six Browning .50s.
(Incidentially, the Ho-5 and Ho-103 are both derivitives of the Browning .50 that the Japanese modified to be lighter and take different cartridges).