1999 Hyundai Elantra P0138, P141, P0335, P0303

I drive my car and it seems to be fine, but if i drive for say an hour and shut the car off, run into the store for 5 or 10 minutes and try to start the car again it will turn over but wont start, for example it happened to me today and i had to wait an hour and a half and the car finally started. I know that the P138 and the P0141 are O2 sensor problems, the P0335 is a CKP sensor problem and the P0303 is a misfire on cylinder 3 but i dont know where to start with respect to fixing the problem....

I was wondering if anyone out there has any ideas as i dont want to buy an oxygen sensor and then a CKP sensor and finally find out it was just a bad plug triggering all of these Codes.

I am currently thinking that the problem is the O2 sensor? Can someone please help?

Just shooting from the hip, but it sounds like a crank sensor. A plug won't cause a no start and an o2 sensor is unlikely because the vehicle would be in open loop on start up and the o2s wouldn't figure in for a couple seconds anyway. Scope the crank signal when it won't start. IF you dont have a signal, then check for ref. voltage and ground. If you have that=bad crank sensor.