I changed your LOAD script into a INLINE version, so it is not depending on
external files and other users can copy and paste it easy.
My LOAD script:
Data:
LOAD * INLINE [
column1, KeyColumn
1, Product_A
2, Product_B
3, Product_C
];
OUTER JOIN
LOAD * INLINE [
column2, column3, KeyColumn
10, 100, Product_A
20, 200, Product_B
30, 300, Product_C
];
I use the outer join to be sure that all values for KeyColumn are in the resulting table.
In the dashboard I load all available columns and get:

Next step is to iterate over the existing values and add the three columns and store them in column4.
LOAD
Num#(column1 ,'##.###') + Num#(column2 ,'##.###') + Num#(column3 ,'##.###') as column4
Resident Data;
Attention. This
column1 + column2 + column3 as column4
will not work!
After adding the new column to the dashboard you get:

I hope thats what you were asking for.
Update after comments.
In your case
Data:
LOAD column1,
KeyColumn
FROM
[C:\Qlik\file1.xlsx]
(ooxml, embedded labels, table is sheet1);
OUTER JOIN
LOAD column2,
column3,
KeyColumn
FROM
[C:\Qlik\file1.xlsx]
(ooxml, embedded labels, table is sheet2);
LOAD
Num#(column1 ,'##.###') + Num#(column2 ,'##.###') + Num#(column3 ,'##.###') as column4
Resident Data;
should work.