Hi guys,
In the absence of Adduce, please allow me to be the "Judge" on this. I've checked both out, and here's what I encountered...
With Geoff's option... I tried it on an entire column (by clicking the column letter) - and the formatting ONLY extended down to row 27. I also tried highlighting from Row 1 to 500, and AGAIN it only formatted down to row 27.
In checking the Conditional Formatting in the cells below row 27, they DO have the same conditional formatting formula - BUT these cells below row 27 do NOT show the formatting every 3rd row.
Geoff, I'll still give you a STAR for effort.
THE WINNER, however, has to be Glenn.
Using Glenn's method, it applies in ALL of the above noted situations - i.e. an entire column, or a portion of a column. ALSO, one can copy any portion of the formatted range to another location, and the formatting also gets copied.
Congratulations, Glenn !!!

===> STAR.
Regards, ...Dale Watson dwatson@bsi.gov.mb.ca