Pro tips for earning 3 stars on Level 12 of Classic Tower Defense:
1) Put a level 3 Laser Tower at the top left since it will reach the zombies that come out of the start gate, and it will also be essential in eliminating the zombies that come around the top left corner.
2) Don’t neglect putting a level 3 Laser Tower at the top of the map in Wave 5 to avoid any Zombie dogs sneaking through.
3) Start the game with Rocket Turrets in front of the opening gate, and change these to Laser Towers when the Boss comes in the final wave in order to do early damage to him. He’s got a lot of HP so doing early damage to him is important. Otherwise you might not be able to kill him before he reaches the end gate.