As mentioned in this thread Red Hat SSO or Keycloak is the recommended way for using Oauth2 or OIDC with 3Scale:
https://groups.google.com/forum/#!topic/3scale-api-management-by-red-hat/CWTjPVnWY88
I am not really familiar with IdentityServer3, but assuming it is a Oauth2 complient identity provider like you mentioned you can leverage the identity brokering feature of Red Hat SSO/Keycloak to connect to IdentityServer3. By default most of the popular identity providers like Google and GitHub are out of the box supported, but you can add your own. I know this is possible with an OIDC identity provider. Not sure if it will work with IdentityServer3.