Hi Lauren,
I would put all values in a single file with a .1D suffix,
e.g. acf_params.1D. If this has 3 columns (one row
per subject), the numbers could be averaged with
3dTstat (using a transpose operator on the input):
3dTstat -prefix - acf_params.1D\'
Note that '-' as the prefix means to write to stdout,
unless you prefer a 1D output file.
- rick