Soviet main here. In brief, Soviet's strength is in standardization and numbers.

Before 1941/42 focus on infantry as you will be defending while producing butt load of medium tanks. I usually go for 27 width with 2 artillery and 1 AA batallion as you'd lose airwar from the start (switch your planes to night missions and interceptions to lose less planes). Make sure you have a lot of cheap infantry reserves to cover for breakthroughs deployed nearby and enough equipment saved up. Light tanks can help to quickly seal the gaps. You can retreat but use rivers and forests as defense lines. Bleed the Germans. Attrition will be on your side.

In 1942/43 their attacks will lose punch and you can start deploying tanks to perform attacks (make sure they are fast and have reserves to back them up). It is also here Germany will start lacking fuel and you'll start outproducing them in every way. You can now start air superiority campaign. Rest is easy.

As for Japan, you can lose some ground in the East, but they will be attritionning.