Combine three portable graymaps into one portable pixmap
rgb3toppm redpgmfile greenpgmfile bluepgmfile
Reads three portable graymaps as input. Combines them and produces one portable pixmap as output.
ppmtorgb3(1), pgmtoppm(1), ppmtopgm(1), ppm(5), pgm(5)
Copyright (C) 1991 by Jef Poskanzer.