I've started to use T2 as soviets in 2v2, so i can get a early halftrack and penals. The 50 cal on the halftrack can destroy the kubelwagon in 2-3 bursts and penals imo are amazing.
The point is not to add a T34/85 to T3 as it is to add a T34/76 to the HQ to provide more variety and adaptability to the Soviet meta.
Making the T34-85 a T3 immediately would basically rule the T34-76 at HQ useless, it would turn into another t70. There has to be some sort of fuel cost to gain access to the 85.
Why not instead making it an upgrade? keep the T34-76 in the T3 building, to gain access to the T34-85 you have to "research" it for like 100-150 fuel. Once research you can then choose to build T34-85s from the T3 building or the T34.
As of now the T70 is nearly worthless because of the high fuel cost of getting the T3 building. By the time you can get a T70 the germans are getting shreks and AT and possibly tanks. I feel they should move the T70 to either T2 or T1 giving the russians a little stronger early game because as of now the russians really have no way to deal with kubel spam effectively