However it's not impossible that a small number of M4A3 may have been shipped as it says that the Soviets received Shermans up until the summer of 1944.
Regarding the sherman, Yes, there are mentionings of M4A3 beeing lend leased to soviet. So having them in missions would be oki. Im not shure about what guns they had thou, 75 or the 76, i have no idea :S
One thing to understand is that most of the Sherman variants (M4, M4A1, M4A2, M4A3, M4A4) were produced
concurrently from mid-1942 to the end of the war. The variants merely indicate different engines and internals.
To say the least, it would have been an unnecessary logistical problem to supply and maintain 2 different variants for a remote 'customer', especially since the M4A2 was in continuous production throughout.