Here is a solution that produces the following results....
Where cell labelled 'A*B*C' has a FormCalc calculate event as depicted by row[0], below
// form1.page1.subform1.table.row[0].sum::calculate - (FormCalc, client)
var sum = row[0].a + row[0].b + row[0].c
if (sum == 0) then
0
else
Max(row[0].a,1) * Max(row[0].b,1) * Max(row[0].c,1)
endif
Steve