I have a Robert Shaw (Maple/Chase Item 9500) room thermostat for heat/air.

The Temperature reading is 3 degrees high compared to any thermometer that we set by it. Is there any way to correct the reading so it compares with other good quality digital and mercury thermometers?

Is this the temperature that determines (considering the differential) the on/off of the AC?

Some thermostats have an adjustment for the displayed temp vs. the measured temp. If you have the installation manual, read it carefully to see if any such adjustment is available on your stat. You may be able to download a manual if you don't have one.