Note the exclusions as per the T&Cs:
Ineligible purchases include: new Costco memberships (which are eligible to receive 1,000 Bonus Aeroplan Miles for a new Gold Star or Business Membership and 1,500 Bonus Aeroplan Miles for a new Executive Membership), Costco membership renewals, gift of membership, tires, online gift certificates, Costco Cash cards, custom installation services and online digital print orders.
Also believe that registration is required to get the 3X.