I was told if you tap 1 phase of 3 to get 230v mains you mustn't have a load on the other 2 phases and the load should be balanced over all 3 phases.
This is not true you seem to be contradicting yourself in domestic wiring they only take one phase into the house and the next house would get another phase etc this is so the 3 phases are balanced. The same in a factory the phases need to be balanced as far as possible.

The danger can come in domestic use if a guy runs an extension lead to his neighbours house its possible to get the high voltage in fault conditions