To taxi with inboard engines running ( they're the ones with the hydraulic pumps) with a delayed start for the outboards is standard practice. Typically saves in the order of 1000lbs of fuel and is smiled upon by the bean counters.
A Three engine take-off and ferry for maintenance under a special operating certificate with a crew especially trained for the stunt ( things can/do get squirrelly as asymmetrical power is applied on the take-off roll) is also standard practice.
Shutting two down enroute increases endurance but knocks the specific fuel flow down south, absolutely no commercial advantage to it. The bean counters would soon start screaming.
A DC-8 will stay airborne with just one engine running - at least in the sim. If shutting down two were to make money think of the savings to be had with three blowing in the wind.