I would say that the most effective method would be to write a stored procedure to do all of this for you.

If that is not an option, the next viable solution is a subreport for each column...though this might slow your report to a crawl.

Why? In order for you to get the last year of data into the report, the effective dates become a year earlier, and all of that data will probably mess up your report as you would get more month columns (12 more). By using a subreport, you can display pull as much data as needed (but it's in another report, so you don't see it) and display the result.

Thinking about it, you might be able to get away with just 1 subreport for all 3 columns, though I am not positive about that...originally I had thought 3 subreports, 1 for each column.

Hopefully one of these ideas will lead to a solution...or someone else will suggest another path.

I don't think that you need reset...after all, what if your last record is from 2 years ago, then your total would be 0. I am not a running totals guy. It would seem that you have coded everything that you need in the formula, you could just display the result at the end of report.

I think what I would do is:
Put a Record Selection Formula into the report that would be the 12 month logic that you used in the formula. Now that the only records in the subreport are for the prior 12 months you can just do a SUM() on the field you want and report that

HTH

first question, do you have a filter on the report to only select data for a certain date range? if so, does it go back 12 months? if not, the formulas won't work as there is no data to populate them with.