Excel 2007 : OFFSET formula

Hey all.. newbie here.
I have been reading through threads and haven't found one that applies exactly to what I'm looking for. Forgive me if it exists somewhere and I have not searched correctly.
I am trying to find a way to generate a formula that can be copied across every 3rd column, but will automatically give the result that is 2 cells below the last formula.

Offset problem.JPG


This is the formula I used to get the first value:
=OFFSET($L10,-8,-10,1,1)

However, I need to be able to copy this formula across (I have many cells to populate).

Let me know if more details are needed. Thanks for any help anyone can offer.

Jesse