I have seen 800 amp services using two 500 kcmil conductors for each phase. I'm assuming that the ampacity is figured as 430 amps(THHN) X 2 = 860 amps, rather than as being equivalent to one 1000 kcmil conductor at 615 amps. Would three 250 kcmil parallel conductors be 290 amps X 3 = 870 amps?
One little detail to watch out for is to be sure that the noncontinuous load plus 125% of the continuous load does not exceed 760 amps for parallel 500kcmil. This is usually not the case since most of us don't cut these things real close to the bone & leave some room for future growth.
Few things are harder to put up with than the annoyance of a good example.