I have calculated that the penetration of the 3" 16cwt gun would be a little bit better than the American 75mm gun on Sherman and Chaffee (about 10% better) and somewhat inferior to the 76mm gun on the M-10 and M-18 (about 25% worse).
I asked about the 3" AA guns anti tank performance on Tony Williams site Military Guns and Ammunition http://forums.delphiforums.com/autogun and I was told the 12.5 pound AP shot would penetrate 86mm at 1,000 yards. Not sensational but probably enough to kill any tank in 42/43 at a good range apart from the Tiger 1. Plus it would fire a useful HE shell.