How long is it OK to leave a car?
How long is it OK to leave a car? There is no straightforward answer to this, as it depends on how well you prepare your car for storage and how often you maintain it. Generally speaking, you don't want to leave your car untouched for more than two weeks. Store it for less time than that if you're especially concerned about your battery draining.
Is it bad to let a car sit for 3 days?
If it is a newer car, this simply doesn't matter. If your car is less than 3 years old you can leave it alone as long as you want and the only thing you might have do deal with is a dead battery requiring a jump start. Of course, your car would be better off after the three years if it was driven periodically.