One or three boilers for a multi unit?

I have a 3 unit I live in located in Portland, Maine. I pay for the heat for the whole building since there is only 2 furnances.One furnace is a old steam unit that is about 50 yrs old that heats the 1st floor unit I live in. The other furnace heat is hot water baseboard (about 30 yrs old). It heats the 2nd & 3rd floor unit. So I have to pay for both the units heat since there utilities are not separated. The heating bills are killing me. I have replaced most windows in this 100 year old house, and will replace the remaining one this summer. I'm trying to decide if I should spend the money and purchase a boiler for each unit so they would pay for there own heat & hot water. Or should I buy one furnace that would take care of the entire building? I plan on staying here for a long time, so my gut feeling is to get a separate boiler for each unit. I have quotes being given to me for combi units for each apartment. For the 2nd and 3rd floor units, one contractor suggests a baxi HT 330, the other a biasi riva combi with has a 15 gallon integrated indirect water tank. I think these are both condensing boilers; both are for heat & hot water. Each unit is about 1600 sq ft with one bath. For the 1st floor unit, which I live in, I want to continue to use the nice cast iron radiators. One contractor thinks I can change them over for hot water use with a Prestige Solo condensing boiler with an indirect fired SMART Phase III water heater. My unit is about 1600 sq feet a well.
After look through the site here, it looks like many in the Northeast are recommending the Buderus boiler with a mega Store indirect water heater. It seems they have a branch in NH. jadnashua , the moderator seems to have gone this way. Any advice for other solutions for me to look at would be appreciated.