I think T3 is only good if u get ahead, so u can finish the game fast. If your opponent played the early game well, and u couldnt get a big enough adventage, and he starts to turtle in one side of the map, go for T4.
Hold his tanks with 1-2 AT-s, maybe one SU85, then go for 1-2 Katyushas it will do a ton of damage and makes turtling very hard and costly to maintain. Pros sometimes go for a fast SU76 first after T4 finishes, its pretty strong vs anything the german will have at that moment (and u can barrage mg's and at's to death).
if you can micro well enough su76s can be very powerful. esepically agianst at guns and bunkers. but also against all tier 3 units.
i think right now its mostly best to avoid tier 3 this patch anyways. (though the quad car is fucking amazing
)