you wont be able to have more than two AT guns in a contested game, and you wont be able to defend them with static globs, you would lose the whole map.
You have to shoot and retreat them or pull them up with an offensive and then pull them back again, mortars flame and otherwise or offmap artillery would make short work of static units and your opponent will end up with your AT guns.
If you have 3 AT guns by the time your opponent gets the first ostwind out you are doing it wrong I agree. I'm talking about in general when a game goes long and there's a lot of armor or something big like a tiger on the map. I still think 2 AT guns is reasonable if you are down on fuel.
You misunderstand what I am saying about the AT guns. They should always be on the move right behind the infantry. Just select the AT guns as a group and target enemy vehicles manually when they are distracted by your infantry. The damage of 2 AT guns together is sometimes enough to kill a tank before your opponent can react. I find it's much more effective than a single AT gun that can at best just drive your opponent's tank away.
Also, just as something to try... I have successfully used an SU-76 against an Ostwind and won before. You have to be very careful with it though and keep it back so it won't get flanked but it does have a range advantage and enough frontal armor to deal with that 40mm gun. A T34 is probably a better option but just something to think about.