Yes, you can!

Try this:
1) Create a workbook with three worksheets.
2) Type some data into the first one or two worksheets.
3) On the third worksheet, pick a cell and type "=". Do not hit <Enter> yet.
4) Leaving that formula unfinished, navigate to another worksheet, either by clicking on the tab or just hitting <Ctrl-PgDn> and <Ctrl-PgUp>.

Note that in the formula bar, your formula is sitting there still waiting to be completed.

5) Once on the selected worksheet, select a cell.

Note that in the formula bar, Excel has filled in the address of the cell you selected.

6) Hit <Enter>. The cell on your third worksheet now points to the cell on the other worksheet. You can use this syntax to combine values on other cells in various ways, just plugging them into formulae. You can even do it from other worksheets, if you want to, though that is not always convenient.