#!/bin/tcsh

set dset_roi = THE_DATASET_NAME

# get the 6 extents in terms of IJK slice indices
set extent = `3dAutobox -extent_ijk "${dset_roi}"`

# select out the min/max along the AP axis
set apmin = ${extent[3]}
set apmax = ${extent[4]}

set div1 = `echo "scale=0; ${apmin} + (${apmax} - ${apmin}) / 3" | bc`
set div2 = `echo "scale=0; ${apmin} + 2*(${apmax} - ${apmin}) / 3" | bc`

# report results in terminal
cat << EOF
Slice values are:
${apmin}
${div1}
${div2}
${apmax}
EOF

# apply results
3dcalc                                  \
    -overwrite                          \
    -a "${dset_roi}"                    \
    -expr "step(a)*(1+ispositive(j-${div1})+ispositive(j-${div2}))" \
    -prefix TRIPLE_ROI.nii.gz