You can use Conditional Formatting for this:
If you have Excel 2003 or older:
Select your range of cells to apply the "sight lines" to
From the Format Menu, choose "Conditional Formatting"
Change "Cell Value Is" to "Formula Is"
Enter the formula: =MOD(ROW(),3)=0
Click the Format Button
Choose your format (fill color and/or borders)
Click OK
If you have Excel 2007:
Go to the "Styles" section of the Home Tab
Click on "Conditional Formatting"
Select "New Rule"
Select "Use a formula to determine which cells to format"
Enter the formula: =MOD(ROW(),3)=0
Click the Format Button
Choose your format
Click OK
Note: You can adjust the numbers 3 and 0 in the formula to change how many
rows are skipped, and which row the formatting starts on.
HTH,
Elkar