Regarding not being able to see your units: Go to preferences and turn unit numbering on! This highlights units nicely, and for infantry units tells you how many individuals are there, and all the other units in the same platoon as the selected unit glow orange, and the platoon leader is marked with brackets. Now you can go back and play it properly.
This works in SP:WOW, and I'm sure it is the case in SP3 as well.
My other tip is to put "Scroll delay" on 3 - on modern computers the default XXX speed is way too fast.
Steel Panthers 3 was the game that encouraged me to buy my first PC. Eventually I fell into the bad habit (in the solitaire scenario generator at least) of just having 1 HQ, 1 ammo dump, 1 or 2 scout helicopters, and spending the rest of my points on MLRS batteries.
What are people's opinion on the "C&C" mechanic in SP3?, do people play with it off or on? I remember finding it a bit confusing, but it's 15 years since I played it, I must dust it off and give it another go.