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Soviet old tech system was barely holding all together due to infinite no tech requirement vehicle call ins.
Once people started to optimize gameplay, they basically remove the weak link which was teching and they basically skipped going for tech which resulted in either KV8 into IS2s or 2x T85s.
Yes, initial T3 with weak T3476 worked due to how broken ram was. Same with Su85 with turbo reverse and main cannon machine gun. IF you go for an army with limited options, then those options were broken. Maxim spam and precision strike mortars. Or double man squad sprinting snipers inside a clowncar.
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The problem is that the game was not designed from it's core as a team game based RTS. It allows to be played in that way, some balance aspects take into consideration team game things, but there's so much you can do with what you have.
Outside of really specific moments in the game life, the main problem for teamgames (3v3+) has always been unit volume, maps and match making.
As far as popularity, i think in order it was 4v4/2v2 > 1v1 > 3v3.
In regards to how balance should be done, EVERY COMPETITIVE PVP game goes from top to bottom. There's a difference between balancing for your best players and down the line and completely ignoring the noobs input. What "bad" players needs is not balance changes, rather than QOL and ease of use patches which makes their experience with the game better. What you aim for is making the game "easier" while not lowering down the ceil floor.
Ex: Reverse hotkey, compared to how it required to be done in vCOH1. Some things that could be done for COH3 could be auto reinforce toggle on/off in base.