I loved PG 2 and spent hours on it. I could lower the difficulty enough to enjoy it with hoards of jet bombers and super battle tanks to storm off the maps in endgame if I wanted.

I had been very disappointed in PG 3D back in the day for several reasons. The ugly maps (compared to beautifully hand drawn PG 2 ones), buggy 3D technique (I stopped playing due to a bug corrupting the textures), and too few units to play to cover the map.

Now I wanted to see if the 15 years older me would enjoy it better. The answer is no. Because at the easiest of easiest level of difficulty, I simply lost at the THIRD mission of the French campaign. The enemy is spawning units non stop, deploys an endless number of tanks and uses the very limited number of units to infiltrate them on your backs. There's always a tough-as-hell Panzer to take care of somewhere before it ruins your weaker units.

Ironically I never had problems with the difficulty in PG 3 but I got frustrated in SE after a while (with the Germans at Kursk).
I'm trying to get 3D to work properly to try it again and see if it was just better or I'm really just getting old since I'm also getting frustrated with the difficulty of modern TBS (Order of Battle, Panzer Korps) but as I remember PG3 plays very differently to 2.

Neither you nor the opponent can run out of units easily since there is no prestige to worry about, only slots. What you will run out of will be good units and commanders for them. Since there's only so many (for ex) panzer 4's to go around if you keep losing panzer 4's you'll be forced to replace them with panzer 3's until you lose all of those you have in the depot and than be stuck with panzer 2's and so on. Same with commanders, eventually you'll be down to no commanders thus your units will be able to perform 1-2 actions per turn as opposed to 10 with a max rank commander. Same with the enemy, kill his Tiger and yeah he'll bring a replacement but it's unlikely to be another Tiger and unlikely to have a commander as good (depending on the date ofc). Meaning the point isn't to kill all of his units, it's impossible, but kill enough of them to complete the mission. It's still worth it to kill as many as you can for xp and force him to use worse equipment later in the battle.

Can you be more specific about what's going on in that mission, what's your army composition, are you balancing commander quality with unit numbers (one unit with a rank 10 commander is nice but you could have 2 units with rank 5 commanders in them for those slots)?

I'm going to offer some general advice until you can offer more info:
The game might be called Panzer General but the king of battle remains artillery, make sure you have enough of it as well as infantry to more easily capture the towns. Don't forget to perform your attacks in proper order, move a unit with arty spotter close to the target for extra arty damage. Arty should usually be used to suppress an enemy not kill it so others units can attack without taking return fire. Spread your arty fire around if you can. Don't rush ahead with fast units, keep some action points in reserve for counter or support fire on the enemy turn. Keep your units and commanders alive.

Try watching some Youtube let's plays to see what others are doing.