Sj
One or all of these three companies must have hired the crew and arranged for their training.
Hired, probably. 'Training' however is a bit vague. The investigation (assuming its similar to a UK in content/context) will look at the career path/history of the crew, including hours flown on type etc. Both may have come to the airline with a Metro type rating and hours on type, or may have done the type rating conversion with the airline in house, or a mix of both.
R30
The 'operator' is defined as the person for the time being having the management of the aircraft. In the programme Flightline BCN, named as the AOC holder, clearly didn't have management of the aircraft, they didn't even know what operations it was undertaking or control of the crew.
The flightplan will need to have been filed with CFMU/Eurocontrol, and the airframe and crew will need to have been 'allocated' to the flight by some one/organisation. The crew will have been rostered and known to report at airport X to brief and fly airframe Y to airport Z. To fly the schedule the organisation managing the flying programme will have known and scheduled to have an aircraft at Belfast to fly the route. Therefore there will be an easily identifiable organiser, (to the investigators), of that basic requirement to operate that schedule.
Fuel etc and handling issues will need to have been dealt with on a daily basis and the aircraft will need to have had line maintenance by an organisation too. Those personnel and organisation/s organising the essential requirements above, (a/c alloc / crew / handling / Mx), would likely be considered to be undertaking the 'management' of the airframe and crew.