Tylenol extra strength prescription is 2 tablets every 6 hours....so is it safe to take 1 tablet every 3 hours?

You can do that if symptom is in better control that way. However, it is safer to limit tylenol use 2 gm or less a day if you plan to use more than 3 days. Take care.

If my arithmetic is correct, one 500 mg Tylenol every 3 hrs amounts to a toxic amount of 4,000 mg daily. In an 83 yo the risk of liver damage is sky-high! Contact your local doctor and discuss this further, prior to proceeding.