When would CAPT 3 or FO 3 ever be used on the A320?
"ECAM" is a typical terminology of Airbus aircraft.

From this accident report found quickly while searching for "CAPT3":

The panel:

  

The explanations:


  Normally all switch are positioned on NORM selection, mean that all of
  the source  are coming from co-location sources (i.e. system 1 for
  Captain, system 2 for F/O and  system 3 is standby).
  
  In case of
  failure of either of the related system sources for Captain or F/O
  side, they  can alternate it by selecting the switch to either CAPT 3
  or F/O 3 (CAPT or F/O for  ECAM/ND XFR).


The switches are used to designate which input to connect to equipment or display.

When everything is fine, the NORM position links a left side input to a left side equipment, and a right side input to a right side equipment. If an input is not operational, then the switch can be used to select the third input for either the left side or right side equipment.

The equipment and sensors we are talking here are:


Attitude / Heading reference
Air data reference
Electronic Instrument System / Display Management Computers
ECAM and ND XFR used to manage where appear PFD, ND and SD.


Detailed example

The aircraft attitude and heading are sensed using cooperating inertial sensors and air sensors which are combined in an ADIRU. As the attitude determination is key to pilot the aircraft, in particular when autopilot is used, there are 3 redundant ADIRUs on a A320, one for the Capt (#1), one for the F/O (#2) and a backup one (#3). Each ADIRU #n receives data from its ADR #n unit (air data) and IR #n unit (inertial).

The NORM position allow to send the left ADIRU to the Capt panels and the right ADIRU to the F/O panels. This allows the Capt and the F/O to have separate data, and they can compare them in case of doubt. Same for the flight control computers behind Airbus fly-by-wire.

If ADR or IR fails on the Capt ADIRU (#1):


ATT HDG on CAPT 3 position replaces IR #1 by IR #3 on the ADIRU #1
AIR DATA on CAPT 3 position replaces ADR #1 by ADR #3 on the ADIRU #1


If the failure occurs on F/O side, corresponding F/O 3 positions would be used to replace IR #2 and ADR #2 by IR #3 and ADR #3.