I payed for the 3 months premium for $9.99 on June first and it ended yesterday. I remember specifically buying it on a deal because I had previously cancled my premium and restarted it because I saw the discount.
To get the 3 month promo you must have left Premium before April 19th, and not have used a similar promo in the past.
It's not possible to get one of the offers with a payment method that's already been used to purchase another offer before.
If that doesn't apply to you, contact customer support through this link:
They'll help you out. They take, at the most, 24 hrs to get back to you.
Don't forget to check your spam folder for responses as they may end up there.
If you get an automated email directing you to the community or help pages, you need to reply directly to firstname.lastname@example.org and one of the customer services agents will get back to you.