Answer: Enable universal group membership caching at sites 2 and 3
Configure the domain controllers at sites 2 and 3 as global catalog servers
Step-by-step explanation:
Since the user wants to improve login and resource access performance for users in sites 2 and 3, then a universal group membership caching ashould be enabled at sites 2 and 3.
Also, the domain controllers at sites 2 and 3 should be configured as global catalog servers. This is required for authentication and the resolution of the user principal name. This can be used in ensuring that login is improved for the users.