The problem with the SU85 is that its not even a SU85 to begin with.
SU85 had T34 armor with a T34 85 gun.
So 45mm of armor.
The SU 85 we have in the game has 75mm of armor, because its a “SU85-M”
If it had 45mm of armor, it should absolutely sit in BR3, because not only because it is a SPG - but because it has no MG what so ever.