In the distant past I flew a 3-eng ferry flight in a Vanguard, also sim trained for 3-eng ferries on both the B747-200 and A340-300
During these exercises the two engine inop performance is taken into account. Not surprisingly this results in very restricted take-off weights and high V speeds. At the normal weights used on these aircraft it is therefore fairly obvious that only multiple PARTIAL power losses and or sufficient height to trade will make such scenarios flyable.