When the functions have more than 2 variables I can not assembly a matrix to find the max and min values.
Yes, unfortunately Prime does not support multidomensional arrays.
So as I see it you have two options:
1) Create a different datastructure, e.g. a onedimensional vector (similar to whate the use of range variables would yield). You may use a tricky way with the three range variables or a small programm (preferrable) to do so. I guess a matrix with 4 columns (the three arguments and the function value) and (in your case) approx., 3 millions rows would be a good choice. You would use three nested for loops to do so. The use min() and max()
2) Write a routine (again three nested for loops) which only returns the min and max value (and if you need the three arguments which yield those values) without creating that huge structure.