In a helper column adjacent to your set of data, you can enter a
formula like:
=MOD(ROW(),3)
and copy this down. This will return values of 0, 1, and 2 down the
column. If you apply an autofilter to this column you can select which
of these values you want to display - suppose you choose 0. Then you
can click on the first visible cell, hold down <SHIFT>, press <End>
once followed by <down>, and then release <SHIFT>. Without clicking on
another cell, use the pull-down on the filter to select All, and then
you will have every third cell highlighted.
Hope this helps.
Pete