To redeem the offer, download the app and navigate to the "Account" tab. In your account, click the "Offers" link, and it'll pull up all three deals. You can start an order right in the app or click a link that will allow you to use the deal in-store.
The offer is limited to one per person, but you can go ahead and borrow a friend's phone for your second burger run.
h/t Don't Waste Your Money