Its an old game, and its gameplay doesn't age well.
Zombie Horde 3 is basically a grinding game, and despite having a large city, you'll find that it is not worth exploring the place, especially since the vehicle is so difficult to handle. You won't be able to pause the game halfway; you'll have to go back to the garage.
//How to rush the game:
1) Follow the yellow arrows to do the defend missions. Grind cash for at least a knife (machete if you want to 1-hit kill basic zombies).
2) Save up $20,000. Buy a rocket launcher and get about 30 ammo to be safe. You'll need the rocket launcher to get past all the barricades in the game.
3) When you're ready, follow the blue arrow. You'll be able to enter a whole new area of the city and have access to a new garage. Use this new garage from now on.
4) Continue following the blue arrow until you're told to go to the airport.
Here's the thing. You're not supposed to go to the airport. You're supposed to go into a forest where a helicopter awaits.
Take the main road where the garage is, head right. Destroy the barricade, and then take the first sideroad that leads north. You'll see it on the minimap: http://i.imgur.com/vudyapG.png
Once you've reached the end of the road, just walk left into the forest on foot, and congratulations you've just beat the game and receive 1 million cash.
Mostly for players who want to play this game nice and slow.
1) With the whip, you can 1-hit kill crows, basic zombies, pistol zombies and the evolved zombies(most of the time). With a bit of timing you can kill the pistol zombies before they shoot, but otherwise don't bother meleeing the ranged zombies.
2) If you need to park the car, DON'T drive in reverse! Just slow down with brakes and exit; for some reason you cannot exit the car when you're going in reverse.
3) SMG is okay if you upgrade it, but Assault Rifle is way better. Shotgun is useless. Sniper can pierce multiple zombies in a row. Rocket Launcher can be upgraded to fire 5 shots at a time. I recommend only having up to 2 weapons in each hand since switching weapons is really finnicky - I suggest L for Knife & Rocket, R for Pistol & <SMG/Assault Rifle/Sniper, pick one>.
4) Fighting at night is not recommended; your vision will be reduced to a torchlight seeing what's in front of you, and after the hospital you'll encounter a special hunter zombie that is pretty tough to kill. I suggest simply farming off the regular zombies in the day near the garage and going back inside when the game prompts you to find shelter.