Still I fail to see the mistake.
The US 37mm was powerful for a 37mm and it could penetrate everything the Germans fielded by 1940 and probably most things the Germans defeated in 1940. And if the 37mm would not do they had the 75mm as a backup. But in the end the M3 gave them (by 1940 standards) a tank which was among the best in anti-armour work and at throwing HE. Also with decent frontal armour and a quick production ramp up. For a gap filler, that is pretty good imho. The full 75mm tank was coming anyway and there they even thought of a 105mm HE throwing version.
If you look at the starting point for US armour in 1938, I would say they made few mistakes.