Usage: 3dOverlap [options] dset1 dset2 ...
Output = count of number of voxels that are nonzero in ALL
         of the input dataset sub-bricks
The result is simply a number printed to stdout.  (If a single
brick was input, this is just the count of number of nonzero
voxels in that brick.)
  -save ppp = Save the count of overlaps at each voxel into a
              dataset with prefix 'ppp' (properly thresholded,
              this could be used as a mask dataset).
  3dOverlap -save abcnum a+orig b+orig c+orig
  3dmaskave -mask 'abcnum+orig<3..3>' a+orig

Also see program 3dABoverlap :)

++ Compile date = Sep 17 2020 {AFNI_20.2.18:linux_ubuntu_16_64}