For all but a handful of levels, getting 3 stars is wholly based on the number of surviving zombies. In most levels, you can't have any zombies die; there are a few later levels where one to several zombies can die.
For the remaining levels, 3 stars is based on how quickly you finish the level. You can see the criteria when you finish a level. Nevertheless, it would benefit you to try and finish each level as quickly as possible so you have less work to do for the speedrun achievements.
The hardest part for me was getting all of your zombies to survive running past turrets. Watch the video closely to follow positions and movements for a greater chance at success.
Credit to TemporalWizard at xbox achievements for the Road Map, Silent Mugger for the Summer Level Walkthrough, and ProDivision for the Winter Levels.
*It seems like the Summer and Winter levels have potentially varying amounts of people/zombies on some levels - the video strategies remain sound with a small caveat for the final Winter level. On the final Winter level, your last attack is against a group of soldiers circling civilians. The video shows 6 soldiers circling, but the game only has 4. I found more success when I hid my existing zombie horde behind the house and occasionally popped out so the soldiers saw me. I kept popping out and running away until the 4 soldiers were grouped into 2 groups of 2 (other configurations killed my horde too fast). Wait until one group is by the corner closest to you and blitz with your horde and hope for the best. Good luck!