There's a bit more to a three engine ferry than just securing the dead engine, jumping in and blasting off on three engines instead of four !
One big concern is a failure of one of the three good engines on take-off - particularly on the same side as the dead engine so the take-off performance calculations have to be very carefully calculated. (A fairly modest cross wind can make the ferry flight a non-starter)
The crew have to be specially trained and current on three engine technique and the flight has to be authorised by the CAA - there are several airfields in the USA which prohibit Three engine ferries 'cos they don't want their nearby towns wiped out if things go wrong !
The special technique is required due to the need to start the take-off using only the two symmetical engines - otherwise the assymmetric thrust will throw the aircraft sideways off the runway. The power of the assymmetric engine is gradually increased until airspeed and therefore rudder effectiveness, can compensate for the yaw.
If it's done too soon or clumsily you're off the runway, if you leave it too late, you don't get airbourne. If it is done in the RAF they probably give the crew an AFC each !!