New Digital Thermnostat.

I am having to replace the old analog Duotherm Thermostat with a new Digital model. There are no instructions in the cardboard box, only a wiring diagram on the inside of the new "control box" that replaces the old one inside the AC unit.

My main problem is that the new T'stat only uses three wires at the wall control, and the old one has six.

Has anyone else done the swap? Where did you find any information on the installation, or do you have any tips for me?

Thank you