The best boiler for a three-bedroom house has to be the proper size and have efficient fuel utilization and sealed combustion chambers that release the exhaust fumes outside the home. If there is a connection to a city gas line, it should also be a gas boiler for savings.
The size of the boiler is dictated both by the square footage of the three-bedroom house and the measure of heat loss through windows and surfaces such as ceilings and walls. This gives a more exact range of boiler BTUs needed than simply using square footage. Combination boilers used to heat water and a home's air are also often a cost-efficient option.