@Account Closed - thanks for the thoughtful response. I'm curious to know what type of solution would require only $2,000-4,000 per unit to install new heating systems? Let me try to give some context - perhaps that will help us to get on the same page.
The issue with zoning my current steam heating system is that it would require us to run individual return lines from every single radiator (at least 15-20) down to the boiler(s), which would require a lot of labor. Also, you'd have to account for the cost of two new boilers, installing electric, individual thermostats, running separate gas lines to each unit, etc. as well. Keep in mind that it's an old building (built in 1925) with plaster walls, etc. that are harder to get through than drywall, and the cost should start to make more sense.
I've also gotten quotes from multiple vendors including a family friend and HVAC guys who were highly recommended here on BP for a variety of other options including AC/heat pump mini-splits (cheapest quote was $10,500 per unit), redressing the steam radiators for hot water using a 97% wall-hung unit (cheapest quote was $7,700 per unit), replacing the steam boiler with a hot water boiler and zoning it for the three units (cheapest quote was $13,550 plus thousands more for any broken radiators and/or water line issues), and replacing with hot water baseboards with three separate boilers (cheapest was $22,500). I'm even considering installing those hotel-style PTAC heating/cooling units, but even that is gonna run me over $10,000 when you factor in the electrical work that needs to be done to power those things.
All that to say - I would love to be wrong about this, and it really only will cost a couple grand per unit to fix, but everything I've seen points to that being a pipe dream, unfortunately.