prci_definitions: add master_3client topology
    
    Some tests would benefit from using a multi-client topology.
    As PR-CI now supports master_3client, use it.
    
    Fixes: https://pagure.io/freeipa/issue/8026
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Armando Neto <abiagion@redhat.com>