After 10 years of learning a single game engine, I admit that what I believe is simple may seem daunting.
Last I heard from BobR, he was updating PE3, but that was months ago.
The combat script and scenerio format is not changed, but AI scripts that do not work as the scenerio editor instructions lead you to beleave now give an error in the log.
If a unit is in the unit csv, it will be displayed in the scenerio. If the unit is not present, a default unit will be used. The default unit depends on the structure of the unit csv. This is stock PE.
If a gun listed in the unit csv does not exist in the guns csv, an error is reported in the log. This is stock PE. PEx adds 2 entries to the guns csv. Both default to 1 if the value is not found in the csv. The first is shell caliber in mm and it is used to affect the penetration model. The second is shell drag and it is used to affect the shell travel arc. Small arms are most affected with an assumed 1.0 value. I have a limited number of fire tables, so I hard coded a value that must be found by test firing. If you have fire tables, or range scales for multiple amunitions (like German AT guns), please publish or PM me.
why were the German optics better? Because the range for each shell could be found by adjusting the optic rather then the gun, thus the gunner's eye never had to leave the optic.
Ostpak attempted to use maximum object resistance to the original game engine to affect the physics of the world and the logic of the AI. This negitivly affects PEx because we added physics based upon what the PE tools could add to the world and the 3D models. Slomo and I went through the original PE models to correct collision boxes, material attributes, etc. If you can not knock over a tree in PE3, try replacing it with the PP2x version. Ostpak adds many world objects, so they must be adjusted in the PE object editor. Tree types now have unique spotting and shell impact modeling (lollipop, giant pine, christmas and umbrella).
Sorry, I ramble. PE3 uses PEx 2009 code. You can simply apply the new exes and dlls, but the bullistics will be affected by the new rules, as will the collision physics.