on the 'Stats' tab of the attached sample the formula in M3 is designed to display the average of the last two numbers entered in cells P3-AAD3. this was supplied to me by someone on this site, and it works exactly as needed. However, our parameters have changed and now I need M3 to display the average of the last three numbers instead. I've tried to change the formula but it results in an error. The error remains even if I try to change the formula back.
I would like not only a solution but a bit of a tutorial on the formula used (or a source to find out how it works on my own). Any help is greatly appreciated.

sample.xlsx


Brian