I don't get it; some of the thinking. To be fair I've only flown 2 engine ETOPS, so know only the rules appertaining to that. The route is allowed to be within 3 hours engine out = 1 engine from a suitable airfield. On a B737 that is 1200nm: on B767 it was slightly further. Those are of the slower modern a/c. I'm not aware of the 'ETOPS?' rules for an engine out on a 4 engine a/c. If a 2 engine a/c, suitable equipped for ETOPS is allowed to be 1200nm from a suitable airfield, I'm mighty sure a 4 engine a/c can be almost without limits. Thus I suspect that on 3 engines they would then need to be within +/- 1200nm of a suitable airfield to continue. Then if they ended up on 2 engines they would divert to the 'within 1200nm nearest suitable airfield'.
Am I missing something?