My furnace keeps shutting off and blowing cold air and then goes into ignition lock for 3 hours and it blows cold air into the house which makes it quite cold, how do i repair the problem?

There are thousands of different furnaces out there and I don't know which you have. If you do not know either, cahnces are you should be calling a pro for safety sake.

Chances are, it is bad high limit switch. There is a temperature sensor or switch that shuts the gas off and runs the blower if the furnace over heats. Does your furnace have a light that flashes a trouble code? That would help determine just what the problem is. The switch or sensor will be in the heat exchanger area.

If you browse the Heating and Cooling forum, you will see we have managed to help many people safely find and fix such problems. If you do give us the brand and model, somebody may be able to say if it is a switch or sensor and more precisely where it is.