Hi,
it is not necessary to use a parent Queue for your setup.
but let’s say you want to limit all the three queues to 30Mbps/1.8Mbps, then you can create a new Queue name=master limit=30M/1800k, and assign it as a parent for the queues. with this setup, the max traffic used by 3 queues won’t exceed 30Mbps/1.8Mbps.