Otto's reply tells you how to apply a conditional format based on a different
cell (ie the format for cells A5, A7 & A8 are based on cell A3), however you
may not be able to apply the border style that you want.
I don't know what version of Excel you're using, but in Excel 2003 there are
limitations to the kinds of broders you can apply with conditional
formatting. You can only apply thin (not bold) borders on the cell edges. The
diagonal "border" that I think you're referring to is not available to use
with conditional formatting.
You could instead apply the strikethrough font style, or else put a hatch
pattern or grey colour in the cell (ie make the cell appear "greyed out").
Both of those are available as conditional formats in Excel 2003.
You could use VBA (macros) to do the diagonal border if you're keen. You
could write code which applies the chosen border style based on the
condition. To get it to apply automatically when Cell A3 changes you would
need to put the code in the Change event code for that sheet. If you want
more info on that, ask.
