So I am trying to regress a .1D file on another .1D file , but the output is actually in .3D ASCII format even I explicitly requested a .1D in -prefix .
Based on the help manual of 3dTfitter, i should've seen a true .1D file (only numbers in column formats)

---------------
OUTPUT OPTIONS:
---------------
-prefix p = Prefix for the output dataset (LHS parameters) filename.
* Output datasets from 3dTfitter are always in float format.
* If you don't give this option, 'Tfitter' is the prefix.
* If you don't want this dataset, use 'NULL' as the prefix.
* If you are doing deconvolution and do not also give any
'-LHS' options, then this file will not be output, since
it comprises the fit parameters for the '-LHS' vectors.
-->>** If the input '-RHS' file is a .1D file, normally the
output files are written in the AFNI .3D ASCII format,
where each row contains the time series data for one
voxel. If you want to have these files written in the
.1D format, with time represented down the column
direction, be sure to put '.1D' on the end of the prefix,
as in '-prefix Elvis.1D'. If you use '-' or 'stdout' as
the prefix, the resulting 1D file will be written to the
terminal. (See the fun fun fun examples, below.)

Is there any availabe command to conert a .3D to .1D format.. Or am i missing something here?

THanks,