It is not 3, but only 1 domain per customer. I know many reported they registered 3 domains, but they could be cancelled at any point. The conditions from their website:
* Offer details:
Sale price applies to new 1-year registrations only. Does not apply to renewals, transfers, multi-year registrations, premiums, or previous purchases.
Limit one per customer, offer valid for new customers only. Price subject to change.