Here's the deal....the short answer is No, regardless of what Sidney or anyone else says about competence. Each system has a different requirement and here they are...
ISO 9001 - Top management shall appoint a member of the organization's management who, irrespective of other responsibilities,
ISO 14001 - The organization's top management shall appoint a specific management representative(s) who, irrespective of other responsibilities,
OHSAS 18001 - The organization shall appoint a member(s) of top management with specific responsibility for OH&S, irrespective of other responsibilities,
If you look the QMS rep must be any member of management (as defined by the organization); the EMS rep can be any individual the organization thinks is appropriate; the OHS rep must be a member or Top Management.
Now if the desire is to:
(1) have a truly integrated system or systems, and
(2) have only one management representative,
Then that management representative must be a member of Top Management, because with integrated systems the most stingent requirement of one system becomes the requirement for all the systems (this is of course only applicable for those common requirements like document control, internal audit, records managment and management representative to name a few)