[1] Modern airliners only require two pilots, the captain and the first officer.
[2] Before a pilot is fully qualified to operate as first officer, they will typically act as a second officer, sitting in the right hand seat, while undergoing training and supervision from a training captain.
A safety pilot will sit in the jump seat to monitor the junior first officer and the captain.
[1] In Ryanair UK, pilots are promoted from second officer to junior first officer when they achieve 500 hours of flight time.
[16] Modern airliners only require two pilots, and do not have a flight engineer or a navigator.