Except in very small sizes, 3 phase motors are generally cheaper, more efficient, more reliable, and over a few horsepower more readily available than single phase motors.
In premises with a lot of machinery, that alone is a good reason to obtain 3 phase service.
In the case of large premises, internal feeders within a building will be cheaper in materials for 3 phase than for single phase, for a given KW loading.
In most places, over a certain load, the utility will normally only offer three phase service.
The limit for single phase service varies, but there definatly is a limit.
Three phase is effectively unlimited.
The cost per KWH is normally about the same. Tarrifs vary from utility to another, but the cost per KWH is normally the same.
In the UK, if a customer has only a small load, but still requires 3 phase, then a small surchage may apply.
In the US, 3 phase for smaller premises is often 208/120, this system has its merits, but there is also a drawback as regards heating and cooking appliances, most such are primarily designed for 240 volt service and will give reduced output or performance on 208 volts.
A hot water tank will take longer to heat the water for example. (the cost of heating the water will be virtually the same)
of electrical construction uses VA.