There is, not literally, but figuritavely, tons of reasons why there are road test rejections.
But, since you only asked for three, here it goes:
1. Having an improper, expired or no inspection sticker.
2. Any defect/condition that makes the vehicle look unsafe or hazardous for when on and off the road, such as poor brakes, unsafe tires, and the list goes on and on.
And finally,
3. The vehicle has an unclean, or unsafe, interior.