Keep in mind that the offer is available for user's that haven't had Premium in the past. If you already had a Premium subscription, your account isn't eligible for the offer.
However, if it's the first time your getting Premium, you should be able to get the offer if you subscribe in www.spotify.com/premium. Make sure that your payment method meets these requirements and that it hasn't been used to pay for another Spotify account in the past.
have you tried googling ‘Spotify premium 3 months 0.99’? if so and it didn’t work, you may not be eligible for it if you have tried premium before or done a free month trial before. hope this helps! :)