My understanding is that CX would have to give us 48 hours in the hotel. This would seem to negate the man power savings from the resultant 3 crew operations, so it seems to me like a huge waste of time.
If they do it to based crews, they can roster 3-crew with little rostering change. Many based crews have a EXB in HKG anyway. In LHR, S/Os have been getting longer patterns than Captains and F/Os most of the time so they are already meeting the requirement of the 3-man crew. I do not think it is a coincidence that the two cities mentionned for the 3-man crew are the only two European cities with bases.