Suppose that for each die, the probability of landing on any of its four faces is equal.
Since we are flipping three different dices together at once, I will assume that the probability of each die getting a certain face is not affect by that of the two other dices.
Hence, we have:
P(three 4s)
= 1/4 * 1/4 * 1/4
= 1 / 64