Not sure what they are telling you or why - I'd be getting more info. A 3-phase digital smart meter does everything that 3 individual meters might do - it allows reading from each phase so why they would insisting on taking you backwards who knows? If you are fitting a new box (not just replacing the old board) then I understand it has to be min ~600mmx~600mm so can't see why 3 meters wouldn't fit - but I can't understand why they would be demanding you do that. Are you sure it is a digital smart meter? Sounds to me like you might be misunderstanding either what you have or what they are telling you (or both!).
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.