Standing for Command/Control/Communications Computer, the C3 aids in interfacing the battle computers of any allied BattleMechs, allowing better integration of tactical information and targeting/tracking capability. If you are carrying the Master C3 unit, as oppsed to the Slave C3 units which can be installed on your lancemates' 'Mechs, you can bring up the map display and see an overlay showing the information present on your lancemates' radar displays, even if they are out of radar range of your 'Mech.