Skipping rows, copying, subtracting...

Say I have Rows 1 through x and Columns A through AQ. Row 1 is the name of the columns, so ignore it. I would like to do this:
For Columns A, B, D, E, F, G, H, I: Take Row 2 and copy it to Row 4
For Column C: On row 4, put "LastHalf"
For Columns J through AP: each column equals that column on Row 2 minus that column on Row 3 (for example, 4,J = 2,J - 3,J).

Now where this become difficult (for me) is that I want this to repeat for every third row. So Row 4 needs to do this based on the two rows above it, then Row 7 needs to do this for the two rows above it, then Row 10 needs to... you get the drift. Every third row (all of them will be blank) needs to automatically fill in based on the two rows preceding it.

I know this is a lot to ask, but I'm desperate...

Once you setup the intial 3 rows according to your specifications (minus the headings), all you have to do is to copy range A2:AP4 and paste it at the next available "A" row where there is no 0 and repeat . Because the cells references aren't absolute you should have no problem. See attached.

Don't see any attachments...