My aprilaire 700 call dealer is flashing and the display is saying E 3. What do I need to do to fix it.

Has this unit ever worked? Do you have it setup with an outdoor sensor? Usually the error E3 message is related to the ODT sensor. If it is operating in manual mode, then there is a resistor that goes across the ODT terminals on the humidistat. Check the humidistat and make sure the wire to ODT terminals are good, or the jumper is installed. It could be a bad sensor.