[abunabil]

The green light pulse 3 times at a time, don't know what that means, it doesn't fire up, I switched the gas and power off and on , nothing happened, any ideas??

 

[diyorpay]

http://www.askmehelpdesk.com/heatin...-furnace-still-flashing-3-flashes-601743.html

Could be pressure switch. Large silver disklike item with hoses attached.
Carefully disconnect hoses and blow into them to clear. Some people use a paperclip if really gunked up.

Check for obstructions to air intake and exhaust.

Should be posted in HVAC section for more results.