With regard to a 3 engine ferry, we know it can fly and land with 3, so surely it's simply a matter of can it get off the ground on 3 when empty-ish?!?! Should be a simple paperwork job ;-)
Some of our guys were (and probably still are ) certified to do it on the 747/744 but it involved the handling pilot (usually management ) having done the appropriate sim work up and some slightly cunning procedures that had no doubt been certified by Boeing by virtue of flight testing. There was always had an inert engine, or at least it's core, on the appropriate pylon.