had to change my gas furnace but now i have a problemn the new unit has white rogers model50a50-473 where the thersomat goes and i only have three wires wich is red green and white

Follow the instructions that came with your new unit. if you do not have A/C do not try to hook it up.

From a PM

LAB MAN I WAS CURIOUS IF YOU KNEW ANYTHING ABOUT TRANE GAS FURNACE WITH AC ADDED

The truth is, I never saw one in my life. Likely it is much like my Bryant and all the ones I have helped other people here with. I agree reading the directions is a good place to start. Much of what I know about plumbing, heating and electrical, I have learned by reading the directions.

Next go to http://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html
Northern Heat did a fine job on the sticky. Too bad I haven't seen him for a while.

Then go to the furnace and see what the red, white, and green are connected to. If R, W, and G, connect them to R or RH, W , and G at the thermostat, and the heat and fan will work unless the thermostat requires a common. If so, you can still make the heat work by connecting the green to com at both ends. If you want the A/C to work right, you will have to have 4-5 wires. If the 3 wire cable isn't stapled in place, you may be able to pull a 5 wire cable in using it. If the thermostat doesn't need a common, and you can do without the fan only option, you can connect the green wire to both G and Y at both ends. Red will need to be jumpered to RC

I am editing this because I forgot to mention connecting the green and yellow would turn on the A/C if you selected either A/C and it called for cooling, or Fan On. It the second case, it would stay on until you shut it off or something went wrong. As I said, if you want the A/C to work right, pull a 4-5 wire cable. You could stay safe and warm all winter and do that in better weather.