I have been looking on the forum for a topic about three-year volatility but I didn't find what I wanted.

I have panel data which looks like: Firm ID CFO Fiscal Year
1 0.042 2011
1 0.057 2012
1 0.032 2013
1 0.045 2014
1 0.031 2015
1 0.030 2016
2 0.041 2011
2 0.048 2012
2 0.051 2013
2 0.050 2014
2 0.034 2015
2 0.043 2016
2 0.041 2016


I would like to calculate the three-year volatility (measured as standard deviation over the year t, t-1 and t-2). I have been trying to compute this but it didn't work.
Does anyone know how to do this?

Thank you in advance!