Three actions that a developer must take, in a B2B Commerce store, to accept credit card payments using a client's chosen payment provider are: create a named credential for authentication with the payment provider, create an Apex class that implements the sfdc_checkout.PaymentGatewayAdapter interface, and create a PaymentProviderGateway record for the custom payment provider class. Creating a named credential for authentication with the payment provider allows the developer to securely store and manage authentication information, such as username, password, token, or certificate, for connecting to the payment provider's API or service. Creating an Apex class that implements the sfdc_checkout.PaymentGatewayAdapter interface allows the developer to define custom logic for processing credit card payments using the payment provider's API or service. The interface provides methods for validating credit card information, authorizing payments, capturing payments, voiding payments, and refunding payments. Creating a PaymentProviderGateway record for the custom payment provider class allows the developer to register the custom payment provider class as a payment gateway in the store and associate it with a payment method. Creating a RegisteredExternalService record for the custom payment provider class is not a required action, as this is only used for registering external services that are not related to payment processing, such as tax or shipping services. Creating an Apex class that implements the commercepayments.PaymentGatewayAdapter interface is not a required action either, as this is only used for D2C Commerce stores, not B2B Commerce stores. Salesforce Reference: B2B Commerce Developer Guide: Payment Integration, B2B Commerce Developer Guide: Payment Gateway Adapter Interface, B2B Commerce Developer Guide: Payment Provider Gateway Object