Data point from Canada re Costco and AMEX
Costco in Canada has severed their relationship with AMEX. They have joint with Capital One offering a Costco branded Mastercard and in the stores all versions of Mastercard is the only credit card accepted. That included Mastercards issues by banks in US, UK, Hong Kong (HSBC).
However at Costco.ca, the Canadian online Costco shopping portal all three major cards has been and are still accepted for payment.