I've seen ways to do this with conditional formatting (http://
www.cpearson.com/excel/banding.aspx), but I'd like to do it with VBA.

How do I modify the below macro to shade alternate groups of 3 rows?
Instead of every other row...

Sub HighlightAltRows()

Selection.Interior.ColorIndex = xlNone

Dim Counter As Integer

For Counter = 1 To Selection.Rows.Count
If Counter Mod 2 = 0 Then
Selection.Rows(Counter).Interior.ColorIndex = 54
End If
Next
End Sub