Some time ago a power plant here suffered a lightning strike which destroyed a transformer. A replacement was obtained from a decommissioned nuclear plant somewhere but required heroic efforts to move it to the plant. An employee there explained to me "The original equipment was banks of three single-phase transformers, but this one is a three-phase transformer. It's a lot bigger and heavier than anything they had to handle when they built the plant since it's basically 3 transformers in one box. That's why it's so hard to move it in."
Since then I've wondered what advantage there is to a single three-phase unit vs. three single-phase units suitably connected. Is there some subtlety in the design of the magnetic circuit which makes the heavier unit desirable?