As per Gamespots article titled "Xbox Live Gold Deal Gives You Three Extra Months Free If You Buy Three" by Alex Newhouse on January 29, 2018:
"You can take advantage of the discount by heading over to Microsoft's store here. Upon buying a three-month subscription, you'll receive an email with a code for an additional three months. You can apply this to your own account or send it to someone else."
Ive just bought one but haven't received a free code yet...lets hope it comes later. Sorry, I'm not sure how to post any links.