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.

Water got in from the gutters and rain hitting the siding face, where it couldn't escape.combined with condensation formed on the inside wall cavity from temperature differences between an air conditioned room at 75 degrees and an outside wall over a 100.  

There were 6 welding trucks in one area of the campsite , all out of state tags , the guy across from me was from North Carolina.

I would say that longevity is important and temperature homogeneity.

Most tenants see me in work clothes which means they're covered with paint and caulk stains, probably have some red Georgia clay on them and probably a few burn holes in the ratty tee shirts from welding.

I held the 2 halves of the case together with welding clamps and used expoxy to hold it together.