Okay, think about it this way:
Each dock uses 10,000 energy.
Each Engineer uses 4,000 energy and doubles the production speed of a dock.
So,
3 docks use 30,000 energy.
1 dock with 3 engineers uses 22,000 energy and produces ships just as fast. Also, you only have one build queue to manage rather than 3, so less micro on your part.
and remember, energy costs a lot of resources.