Could be Better
There are some things that would make this game sooo much better.
First, a laser sight for your guns. I know you have it in 2, but it would be handy for this one as well.
Next, kill streak rewards. Since it's a pretty simple shoot and kill stuff game, kill streaks would add a bit more depth to the game.
Also, the ability to upgrade the weapons themselves would be cool. I know you can do this in 2, but it would be cool if you combined that with the differant weapons.
It would also be nice if you didn't auto reload when you empty a clip. It gets kinda old after awhile.
And, a melee attack would be cool. I don't mean anything just extra special, but it would be cool if when you push a zombie back, you could run up and attack it.
Finally, a change of scenery would be cool. Maybe have the player travel around differant areas, with stops to buy weapons and upgrades.
Just a few suggestions, and I don't know *anything* about programming, so I'm not sure if these things even can be done. Still, they would make a third game *very* awesome!