Three phase service voltage
I'm a bit worried about you messing with three phase at this voltage if you don't know the basics, but anyway, phase-phase voltage in a three phase system is root 3 times the phase-neutral/ground voltage. So in this case it looks like 501 volts is your answer. 498V equates to 287V, so it depends how accurately you are measuring, how much the service voltage varies, etc. What sort of transformers are these, for what application? Are we talking watts, kilowatts or megawatts?

No, wait, I've misunderstood the question. I've just realised I don't know what the question even is. Nominal service voltage? These measurements you have, if they are phase/neutral and phase/phase are what the "exact service voltage" is already.

Please clarify.