Hi!

I know how you feel about some of the "help".

Here is what my Help file tells me about 3-D formulae:

Refer to the same cell or range on multiple sheets
A reference that refers to the same cell or range on multiple sheets is called a 3-D reference.

Click the cell where you want to enter the function.
Type = (equal sign), enter the name of the function, and then type an opening parenthesis.
Click the tab for the first worksheet to be referenced.
Hold down SHIFT and click the tab for the last worksheet to be referenced.
Select the cell or range of cells to be referenced.
Complete the formula, and press ENTER.

This is a quick, shortcut way of doing it.
I'll try to translate:

1st line: Click on Sheet 12 cell D13
2nd line: Type =SUM(
3rd line: Click on the tab of Sheet 1
4th line: Hold down the shift key and click on the tab of Sheet 10
5th line: Click on D13
6th line: Type ) and press Enter

This you will surely admit is not difficult? But, as so often, it's the way they tell it...

Alf