Forumla Doesn't Auto-Increment Correctly

Hi!

My problem is...

I have this formula in A1:

=INDEX('Sheet1'!$L107,'Sheet2'!$L107,'Sheet3'!$L107)

Then I have this in A2:

=INDEX('Sheet1'!$L138,'Sheet2'!$L138,'Sheet3'!$L138)

I want to be able to drag down the formula where it automatically fills in the next correct cell which happens to be exactly 31 cells apart.

So the next formula in A3 should be:

=INDEX('Sheet1'!$L169,'Sheet2'!$L169,'Sheet3'!$L169)

But when I drag the formula down, it doesn't keep the correct tabulated formula. It generates the wrong one.

How can I get it to automatically go down 31 cells when dragging down the formula?