Since you have not shown any code, I will assume you are just looking for an approach/algorithm:

• Create a list of files using either glob or a combination of opendir, readdir, grep and -f.
• Use a for loop and open each file.
• Use a while loop to read in each line of the file.
• split the line and keep a running sum of column 3 values. Also keep track of the number of lines.
• After all lines of all files have been read, divide the sum by the total number of lines.