Here's a way to think about it: every time you use brackets to refer to an element in an array, the number of dimensions is reduced by one.
Look at this code:
int[][] numbers = {{1, 2}, {3, 4}};
int[] nums = numbers[1];
Because numbers is an array
of arrays, if we refer to one of it's elements, an array is returned. We started with a 2D array (numbers), then got a 1D array by referencing an element in the 2D array.
Following this logic, to get a 1D array from a 3D array, the general syntax would be:
oneDArray = threeDArray[indexA][indexB];