Assuming the average person already is smart enough to carry at least the following:
-wallet (with id, cash [not just a credit card], ect)
-car keys
-cell phone
-sturdy and appropriate clothing [including a belt]

My three additional items would then be:

1. A good pocket knife/multi-tool

2. A good lighter or fire starting method

3. A water bottle or flask that allows for water to be boiled inside (like a non-insulated Klean Kanteen).