There is only one large boiler in most commercial machines (assuming its a HX - not something like a Synesso)..... and that supplies all the groups. Most power goes into keeping the water hot to produce steam..... very little is used to heat the water flowing to each group.
The heater however is usually 3 elements - so that each can be used on one phase of a 3 phase power supply.
You can use just one, two or all 3 elements - but if you use say 1, it will take 3 times longer to heat up, 3 times longer to recover from each use..... etc.
Some people do this at home (as heat up and recovery times arent generally an issue)..... but isnt practical for commercial use (unless used for a very small volume of coffees....)