There are 4 royal flushes out of 52c5 possible
hands, which is (52 * 51 * 50 * 49 * 48) / (5 * 4 * 3 * 2 * 1) = 2598960.
So knowing nothing about your holding, the probability of receiving 2 cards that can participate in a royal flush and then having the flop be the other 3 royal flush cards is 4/2598960, or 1/649740.