How to calculate inter-annual variation of inflation for a 3 months period? I have the monthly indexes taken from the National Institute of Statistics of my country (below), then I can calculate its inter-annual variation easily:

((index month year - index same month year - 1) / index same month year - 1) * 100

But my problem is that I want to compare this inter-annual variation against Financial Statements that show 3, 6, 9 and 12 months values. Then this inter-annual variation is useless because it shows the price variation only for a month and not for 3, 6, 9, 12 months period.

To solve this problem, should I calculate the cumulative inflation of 3, 6, 9 and 12 months of each year, then calculate de inter-annual variation of that cumulative inflation and finally be able to compare them to the Financial Statement inter-annual variation?

Is there any other way to do this more accurately ?
Without further details, what I'd do is what you said. Take an 3/6/9/12-month average of the price index and calculate growth based on that.