What THREE things would you need to sub-meter in a residence???

"3 Units", mean 3 houses/dwelling. Thanks

In many jurisdictions, the landlord does not have authority to charge for electric use that is not metered individually by the utility. You may want to check to see if this is even legal for you to do. Otherwise, Install multiple meters at the service. One for each tenant.

There are also specific requirements when you have multiple dwelling units. Unless you're going to provide continuous supervision, the occupants need to have access to the feeder breaker. In addition, they need access to all the branch circuit breakers covering their occupancy.

I presume that since he's talking about a general contractor there's a competent electrician who understands the code issues involved.