As with most AFNI programs, you can redirect output to a text file. In tcsh, that is done with a ">" for stdout (standard output) and then the name of a new text file that will be created or overwritten. For example, this is how you would redirect the output of 3dExtrema to a text file. Here I've resampled the atlas dataset I want to use as a mask here to the input dataset, using an atlas in the same space as the input dataset. The "-quiet" output limits the output to be only the numbers and none of the other text in the output.

3dresample -prefix amy_mnia -master func_tempmnia+tlrc -inset 'CA_ML_18_MNIA::amygdala'
3dExtrema -minima -volume -mask_file amy_mnia+tlrc. -closure -quiet func_tempmnia+tlrc.'[0]' > extrema_out.1D

Then using "cat extrema_out.1D" shows the contents of the text file as

1 0.026 -27.41 3.72 -17.65 1 9.804
2 0.039 27.59 3.72 -17.65 1 8.696
3 0.051 -30.16 6.47 -8.65 1 9.804
4 0.193 24.84 9.22 -5.65 1 6.265
5 0.217 16.59 3.72 -8.65 1 9.804
6 0.224 30.34 9.22 -8.65 1 6.265
7 0.347 19.34 6.47 -17.65 1 8.696
8 0.395 -21.91 0.97 -8.65 1 6.842
9 0.404 -19.16 6.47 -11.65 1 6.842