Yes.
Click in the cell where you want to create your formula, start with the =
sign and go on from there. When you get to the stage where you want to use
a reference to a cell on another sheet, just go to the other sheet and click
in the relevant cell. The formula will fill in the references accordingly,
so you'll end up with something like
=Sheet2!B2+Sheet2!B12-'Third sheet'!B3
