Think about the options for the first and second cells. Either neither cell is above n, one cell but not the other is above n, or both are above n. The only time the third cell would be 0 is if neither cell is above n. Since probability of all events must add to one, we can say that P(at least one cell > n) = 1 - P(0 cells > n).

Assuming the values are uniformly distributed, what's the probability of the first cell not being greater than n? What about the second cell? Lastly, how does the probability of the two events occurring together work when the first and second cells do not affect each other?

If you want to say they must be above x and y respectively, you just have to acknowledge that the success rate per cell is going to be uneven. The rest of the procedure is the same.