I do know some people who actual prefer to have this type of heat because most rooms have their own thermostat and can actually save money by lowering the temperature setting in rooms not being used and raise them in rooms that are occupied.

The way I see it, "pretty" gets tenants in, and "inexpensive bills" keeps them in.Here's my quick list: .8 gpf toilet(Niagara Stealth at Home Depot,) Niagara Sava Spa 1.75 gpm showerhead, Watersense-labeled faucets, and LED bulbs from either Lighting Ever(LE) or Hyperikon(go for the 2700 K color temperature, and usually 9 watts or so is equivalent to a 60 watt incandescent.)

Did you find that tenants valued the zone-specific temperatures?

For example, a Smartthings hub with various sensors that would alert me immediately to water leaks, temperature extremes, frequency of furnace filter changes, and access gained to off-limit areas to name a few that come quickly to mind.The problem is that I really don't want the expense of providing internet & wifi, which would leave the renter's wifi as the only way to connect.  

The contractor should have known better than to use thin rock that would move under stress of walking, expansion and contraction of temperature. 

The Heat Ordinance mandates that during cold weather months landlords supply heat to rental units or to any unit where owners do not have individual control of the heat.From September 15 through June 1, the temperature inside a rental residence is required to be at least 68 degrees from 8:30 AM to 10:30 PM, and at least 66 degrees from 10:30 PM to 8:30 AM.Landlords face fines of up to $500 per day, per violation, for each day they do not supply adequate heat.

I can see how your tenants would be especially happy about being able to control the temperature. 

Another thing to consider is that windows are a major source of temperature loss and sun can heat up the property through the windows.

The A/C deal as to temperature in the unit is probably a Habitability issue so if you need to do a quick fix and get some window units to cure the problem do that.

I can see the following as options:Get rid of the steam boiler and install high-velocity AC/heating (seems like the best, but PRICIEST, option)Limit steam heating to a single unit and install electric baseboard heating on the other two units (perhaps the most cost-effective way to go, though I would still have to install separate AC window units)Limit steam heating to a single unit and install ductless mini-split AC/heaters on the other two unitsRepair/replace the steam boiler with another steam boiler, fix any radiators that have issues, and keep the status quo without zoning heat (not ideal - I can already imagine the third floor being one extreme temperature while the first floor is another)I'm really new to all of this and at a complete loss right now for how to proceed.