This is a problem similar to time problems. I am not entirely familiar
with this system, but I think it is 16 oz to a pound and ca. 29 grams
to an ounce.

So we need to convert each weight to the smallest denomination, grams,
take the largest two of the total grams and add them. We can do this in
cell D5 with the *array* formula:

=SUM(LARGE(A2:A4*464+B2:B4*29+C2:C4,{1,2}))

As an array formula, this needs key combo Shift+Ctrl+Enter to commit.

Then, the formula for A5 (pounds) is;

=INT(D5/464)

The formula for B5 (oz) is:

=MOD((D5-C5)/29,16)

And the formula for C5 (grams) is:

=MOD(D5,29)

Explanation: 464 is 16*29. The exact content in grams of an ounce is
not so important for ordering purposes, but it could be if you add two
grams quantities in excess of an oz. So if you want replace 29 with the
exact number and 464 with 16*this number.

HTH
Kostis Vezerides