Gamespot rated the game 3.5 out of 10. Here are some reasons why.
1. Civilizations - Empire Earth III raped out the old civilization model and made control a region of the world (West, Far East, Middle East), and you would show your true identity as you specialized in a branch of military units (I'm assuming that if you were a Western player specializing in infantry units you would end up as Rome?). Their bonuses were based on basic stereotypes of their cultures.
And even then the West and East are structured very similarly; only the Mideast stands out, and that's due to portable buildings. Even then, they all play out very much like each other. They only all stand out amongst each other once the civs enter the Future, when the Mideast gets to show off their cloaked factory and East their genetically engineered supercreatures.
2. World Domination -
The main reason I like the series was for its historical RTS campaigns; they have pretty much disappeared from Age of Empires. I can just play Rise of Nations for this. Even worse, these maps can REALLY end up crazy, as the Olmecs could be in the British Isles, and Asian tribe could be in Spain, and elephant archers could be guarding a lost tomb in Germany. This is just bad.
3. Bad AI - Enough said. WHERE DID TARA TEICH GO!!!
4. Bad maps - The maps tend to have numerous canyons, valleys, and rivers that would make you feel like your crossing narrow corridors instead of forest and plains. You have to patrol every square kilometer of the map to find to your enemy. Units also have pathfinding problems, which I haven't seen since the original Age of Empires.
5. Historical Inaccuracy - Units from different epochs traveling together occured in Empire Earth I (but not II, thank goodness). Now however, with only 5 ages, you may see Gatling guns fighting alongside arquebusiers. Then there's the historically inaccurate units, such as the Mideast's camel that hurts units with a horrid stench
and a hero prophet who calls out a plague of frogs
6. More Things - According to other player reviews, there are no churches, roads, "priest" units (except for Eastern shamans), or wonders. The multiplayer is buggy, and the game crashes often. Then there are the cartoonish graphics and stupid dialogues. What is this, Shrek the Third?
I'm sure somebody who's actually played EEIII can elaborate on this.