I have a gas fired single stage heat furnace and I am trying to install a HoneywellCT3200 progrmable thermostat. The wiring coming off the old thermostat has three wires Red, Black, Green. There is no corresponding letter for the black wire or perhaps I should not be using this particular model, It says its compatible??

Follow wires to the furnace to see where they go. Then wire as per your instructions contained with your new stat.

Just in case you tossed your instructions away see below.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1631.pdf

The furnace may be marked the same as the thermostat. If not, the wire coming directly from the transformer goes to R or RH. The one from the gas valve goes to white. It is possible the third wire comes from the transformer too. In that case, you will need to sort out which should be hot and which is common. The sticky at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html
Will help.

Although in many cases the green is to the fan relay, it may not have been in your old system.