The three main types of boilers are steam boilers, hot water boilers, and combination boilers. Steam boilers are designed to generate steam, which is then used to heat a building. Hot water boilers are designed to generate hot water, which is then used to heat a building. Combination boilers are designed to generate both steam and hot water, and can be used for both heating and domestic hot water applications. Each type of boiler has its own advantages and disadvantages, so it is important to consider the specific needs of the building before selecting a boiler.