PS Query - Substraction in Group By Clause

Hello !!!

I have following requirement. My PS Query is Having 3 columns with following values.

Col1     Col2     Col3

K1         8          2

K1         8          2

K1         8          2

Requirement was to have Sum of Col3. So I modified query using SUM(Col3) as aggregate expression. Now My Output looks like

Col1     Col2     Col3

K1         8          6

Now...I want 4th Column which will be Subtraction of Col2 - Col3 so that output looks like

Col1     Col2     Col3     Col4

K1         8          6          2

I can use VIEW and implement this but I want to know if this can be achieved using PS Query. I have also tried using expression but its not working.