MacBook keyboard number 3 key mechanism not working

None here can say why it isn't working as we cannot inspect the computer.

Apple's hardware warranty is for one year, so you'll have to do the math there.

In or out of warranty, you can make a Genius Bar appointment for hardware service and support using this link: Make a Genius Bar reservation - Apple Support

 Customer Support (800) MY–APPLE (800–692–7753) or on line /getsupport.apple.com/ 

or call AppleCare Support at 1-800-APLCARE (800-275-2273) 

Outside the US: Apple’s global phone numbers: Contact Apple Support 

Long shot - try cleaning it.

How to clean the keyboard of your MacBook or MacBook Pro - Apple Support

Don't try to lift the key or prod between it. Otherwise, as DIJ recommends, book an appointment at Apple to see if they can help.

BTW - "Compressed air" in the Apple link above means the sort in an aerosol can, not the hose from the air-tools at work or the scuba tanks in your garage.