Usually there are four wires (red,white,green and yellow)on the control terminal on a furnace. I came across this Janitrol Furnace which has only three terminal wires (red,white and green) no yellow. My question is how can I connect an AC Compressor controls (yellow and common) if there is only three wires one the furnace?

Tie your yellow wire from tstat directly to wire going to the condenser unit just make sure the second wire to/from condenser unit is tied to the common /blue on your circuit board, this way when the tstat is set to ac mode it will send signal to contactor to pull in