For the sake of your employees I would interject something here. For a personal day off, paid, or an upaid day off it's not really your business how long it takes them to get a tire taken off, what they're going to be doing. For a medical leave, to always require a doctor's note could be a real hardship for employees. Depending on what type of medical situation it is, going to the doctor could be unnecessary and a real expense for the person. And if you are going to fire them, and they have a doctor's excuse for that last absence, they will be very likely approved for unemployment benefits.
When I was young and had children, there were some illnesses and medical situations that just did not really require a trip to the doctor, but made you (or your child) too ill to come to work (migraines, colds, stomach upsets) But in some workplaces, I was expected to produce one for everything. I was even told once to go to my regular doctor after the fact on an occasion when I was out due to illness without a medical excuse for half a day (I had seen another doctor for another condition the other half of the same day.) This was patently absurd. I soon left that place.
In these times of plentiful jobs, a firm reasonable policy that is fairly administered with some flexibility is really a bonus for employee retention.
What I strongly would suggest after my many years of working for the Department of Labor is that you have a clear and well written attendance policy, that you make sure everyone is aware of this policy and that you enforce this policy. Do not allow someone to get into such a problem with attendance that you're questioning whether they're just laying out of work or really getting that tire changed or really sick.