Mike O. <Mike wrote...
I have a workbook with 3 tabs. *The third tab uses the SUM function to sum
the values in tabs 1 and 2. *It's set up so that the cell A3 in tab 3 looks
to cell A3 in tabs 1 and 2.

I need to add a row in all the tabs but the SUM function isn't recognizing
that I've added a row. *It still looks at A3 when I need it to look at A4.
....

What's the actual formula you're using in tab 3? If your formula is
=SUM('tab 1:tab 2'!A3), are you selecting all tabs, thereby grouping
them, then inserting a row above row 3 in all tabs in a single
operation? If not, that's your problem.