Just like single-phase transformers, three-phase transformers step down the higher distribution voltages to lower levels for utilization by customers. Pole-top, three-phase transformers are simply three single-phase transformers connected together in either a Wye or Delta connection. Occasionally, only two single-phase transformers are used to make a three-phase bank but only in specific circumstances.
Pad-mount three-phase transformers look very much like big single-phase units. Larger ones have fins on the back to help dissipate heat.