Maximum probability in a grid

Hi, this is my first post, so hello!

My problem (and my question) is how can I either express the probabilities calculated below as the probability for a single grid-square, OR how I can convert the single grid-square probabilities to the 9 grid-square maximum (as detailed below).

I have a large number of 3x3 grid-squares.
There is the probability of an event occurring within each of the 9 individual grid-squares. E.g. 1, 0.5, 0.001 (range is 1/x, where x is from 1 to 999)
The system I have takes the maximum across the 9 grid-squares and returns only that value.

I hope that is sufficiently clear, but I can provide further details or clarification.

Many thanks in advance, I hope that this is a relatively easy and interesting question!