Read only access to Slaves from Server A
First a clarification. A machine is a client, not a server, not running the mysqld program. B and C are servers, they are running mysqld and they are part of a master-slave topology, they are slaves of another server (just assume is D)

And after that, mysql permissions are the answer. Just define a user in servers B and C and grant only the select permission

On both B and on C

GRANT SELECT ON *.* TO 'name_of_user'@'IP_of_A' IDENTIFIED BY 'pass'