To be able to do such, you would have to be in such a position that in the event of the failure of one of the remaining 2 engines, the aircraft could still fly on one engine - this would be highly doubtful with any reasonable payload in such an aircraft.
The Nimrod at loitering level above the sea has one engine shutdown and one throttled back to idle, while using the remaining 2 to stay aloft - this would enable a situation such that in the event a failure the idle engine could be spooled up to attain that extra thrust, but in normal circumstances fuel is saved, increasing endurance and/or range.