Cost's for ESB works can be got from the ESB. The cheapest way to suply all needs is a pressurised modulating Combi Condensing Boiler, but for all three flats together it would be a circulated DHW system in combination with one central buffer tank fed by the modulating Combi Condensing Boiler.
Costs would be spread either by a.) the m2 suplied per flat or b.) the number of tennants per flat or c.) by direct metering (the fairest method) . There are heating/energy advisors out there, check the golden pages. They would do the entire job: installing,metering and billing.
Check also the management/rent contracts to see what is needed to be done.