T28E1 CGMC is the only version i see viable for the M3, it had a 37mm autocannon with 240 rounds and a pair of 50s.
Here's what you are looking for:
Twin 20mm Gun Motor Carriage T10E11-3
Date of first acceptance March 1944 Total acceptances 110
(109 converted to MGMC M16)
Manufacturer White Motor Co. Crew 5 men
Basically the problem was the 20mm rounds were not in huge supply in europe, AAA units loved the M-16 for its firepower on the ground as well, later in the war Luftwaffe aircraft rarely existed so they simply converted rather then deal with the supply problem.