Why does the furnace not work suddenly after one year and the LED blinks three times

What is the brand of you furnace, it is important to know because the blinking codes are different for different brands. One other thing are you sure it was code of three or could it have been thirteen or maybe twenty-one. You should have a home owners booklet that has the code in it. Or it could be on the front of the bottom door of the furnace.

If you can find a list of codes, it will help figure out what the problem is. Modern furnaces have several sensors and relays that can go bad. Go to the sticky at the top of this forum and check ''Sequence of operations''. Determine where your furnace is failing, and check the sticky on that component.