I think he mean to say that he has 3 generators running in parallel but only one NGR is there and at one time only one of the generator is connected to the NGR. The remaining two generators run ungrounded.
I have seen the same situation in one of the Power Plant for a fertilizers industry in Pakistan. This was designed by BBC.
But I believe, by keeping one generator grounded at one time, you keep the other two in operation even if there is ground fault on the distribution side since all the generators are synchronised.
If the generators are connected to a common bus, as long as one generator's grounding resistor is in service, all the generators are referenced to ground through that resistor.
Using only one grounding resistor keeps the maximum ground fault current down and eliminates circulating third harmonic currents between the units. The typical 10 second grounding resistor normally have no (official)continuous current rating, so harmonic currents can pose a heating problem in the resistor, although this is generally not a major concern if the generators are well matched.
Switching the grounding resistors adds additional cost and complexity, so there are tradeoffs. It is not too common in the US, in my experience, but apparently it is routinely done in other parts of the world.
