Hi Blummer42,
If you're talking about 3 coax connectors in the Universal Docking Center (UDC), 2 are for satellite and 1 would be for cable TV. There's usually a diagram of the connections in the UDC on the side wall or on the door, identifying what's what.
In addition to connection to the right coax connector, inside the coach, there's a signal booster with a small push-button switch and a red LED. For cable TV, the light has to be OFF. For over the air TV, the light has to be ON.
The TV has to have coax connected to the antenna/cable input. The other end of that wire goes to a coax connector on the wall. That wall plate usually has one connector for cable/antenna, and another for satellite.
The TV menu has to be set to CABLE and you may have to do a channel scan.
If none of that works, you may have a loose connection or a poorly crimped connector.