Do you know binomial coefficients? The number C(n,k) determines the number of ways you can choose k elements among n elements, where order doesn't matter. It is usually written .
There are 8*4 = 32 cards less than (or equal to) 9. You can choose 3 such cards in C(32,3) ways.
Among all 52 cards, you can choose 3 cards in C(52,3) ways, hence the probability of flopping cards all less than or equal to 9 is C(32,3)/C(52,3), which is approximately 22,44%.