The current balance on a three phase supply will change when it is used to feed a single-phase load. Transformers are used to convert the output from a three-phase supply to a single-phase load in order to achieve the best current balance on the three-phase supply.
The balance of current achieved on the three-phase supply has a direct impact on the efficiency of the installation. Canada Transformers provides a technical discussion of single-phase/three-phase transformers on their website (CanadaTransformers.com). The type of transformer used to obtain a single-phase output from a three-phase supply depends on the power and voltage rating of the system, as discussed on the Carroll & Meynell homepage (Carroll-Meynell.com).