Taking the Risk
So, inciting a prison riot as cover for their escape came off the table as quickly as it was proposed. In fact, this information led the trio to assume that anything they do that could potentially get them caught probably was a death sentence. The group needed to disappear in silence. Nobody was to realize they left until after they had gone.
Digging the Hole
So, the trip continued to dig. They started by expanding the concrete ventilation ducts to fit a human-sized mass through it. For some reason, the guards allowed the prisoners to practice musical instruments at random parts of the day. They covered the sound of their excavation with the screech of an untuned accordion for hours every day...