Use all the digits 1 to 9 create three 3-digit numbers, which, when multiplied together creates the highest value possible.
(In reply to re(2): How to do!
I am not quite sure what you mean. It sounds to me that you are putting forth an argument of the form:
Given condition A, quantity Q takes a maximum when condition B holds.
Thus, for any condition A' that is similar (semantically?) to A, we have that, given condition A', quantity Q again takes a maximum when condition B holds.
(here, we have specifically
A="sum of numbers is fixed"
A'="sum of digits is fixed"
Q="product of numbers"
B="numbers are all equal"
If that is your argument, it certainly falls short of a verification. It is at best solid intuition.