Three-phase transformer vs. three single-phase transformers
Imagine each single-phase transformer has an 'O'-shaped core. When you make a 3-phase, you combine one side of each 'O'. If the 3-phase system is balanced (equal magnitudes of currents in each leg), then the total flux through the combined core sums to zero -- basically the centre leg is not necessary. In practice, it is usually present -- it helps in case there is some imbalance in the phase currents, or even a shorted fault in one phase. 

So, instead of needing three complete 'O' cores, you just need 3 'C' cores. This saves about half the iron.