The multiplayer was great with dramatically improved AI and big battles, while the single player was awful and too hard.
The single player was just too impossible, and not very fun. Vehicles were, just like the first two games, able to get stuck permanently and very easily simply by moving into the wrong wall. Some objects would also block the player when, from a 3D perspective, they really shouldn't have.
However, Army Men 3 is still the best of all of the Army Men games ever released, as long as you're playing the multiplayer. With dramatically improved AI over Army Men 2, lots of units and weapons, and a variety of gametypes and levels, the multiplayer had great replayability.
The big battles and teamwork that can take place in the multiplayer are very fun to participate in and watch.
However, the controls can be quite complciated.
The 3D Army Men games would have been much better if they had been full of warfare, like the 2D Army Men games. However, the Army Men franchise will never be good until they make an actual war game with ARMIES. So, Army Men 3 is the best Army Men game you'll ever be able to play.