Running regression in a loop in panel data

Hello,

I have to run regression in a loop on a panel data set. My data is of the following format: I have to regress cash_flow over time, where (1994=1, 1995=2, 1996=3). For each observation I have to calculate regression coefficient for past 3 years. Overall, my data spans over 20 years and have over 100 panels. Is there a way to run the above regression in a loop and save the coefficient of regression(slope) into my data.

Thanks,
Shekhar