To see that integration for functions of three variables can be done in six different ways, start "xStep", "yStep", and "zStep" at their minimum values. Now use the [>>] buttons to change each of these three variables to its maximum value. You could start by choosing any of the three to set to its maximum, then choose one of the remaining two, and finish with the remaining one. This gives 3 * 2 * 1 = 6 choices in the order of integration.