Thank you for helping me out.
I have written got focus event procedure with the below expression. It is not working.
I added three extra textboxes txt5, txt6, txt7. and each control source was set to IIF([Txt1] Is Null,0,[Txt1]); IIF([Txt2] Is Null,0,[Txt2]);
IIF([Txt3] Is Null,0,[Txt3]). It...