Single Phase Transformers is normally not done because it is more expensive than utilizing 1 three-phase transformer. However, there is an advantage which is called the open Delta or V-Connection and it functions as follows: A defective single-phase transformer in a Delta-Delta 3 phase bank can be disconnected and removed for repair. Partial service can be restored using the remaining single-phase transformer open-Delta until a replacement transformer is obtained. With two transformers three-phase is still obtained, but at reduced power. 57.7 of original power. This makes it a very practical transformer application for temporary emergency conditions  

three phase transformers are four winding transformers designed for two purposes.  The first is to function as a low voltage isolation transformer with a capacity of .050 KVA to 5.0 KVA.  The other is to correct consistently low or high voltage conditions.  Variation of the input voltage is passed through to the output side of the transformer in the same percentage.