When using the top oven, it will not hold temperature and f3 code pops up. Could this be the thermostat?
This is most likely the temperature sensor. Test it for continuity. It should read about 1100O at room temperature, (approximately 70°). Be sure to tap on the sensor with the meter connected to it in case it is intermittent. You should also test to the sensor housing to insure it isn't internally shorted. From either wire to the sensor housing should read no continuity.
If the sensor tests properly, go to the control and test the sensor from there to verify the harness integrity. If this tests properly, replace the control. To verify that the code is corrected, turn off power to the unit for at least one full minute then re-establish power to the unit. If the code doesn't recur by the time it pre-heats to 350°F, the issue is resolved.