#compdef -p p[bgpn]m*

local pat

_compskip=1

if [[ "$words[1]" = pnm* ]]; then
  pat='*.(#i)p[bgp]m'
else
  pat="*.(#i)${words[1][1,3]}"
fi

_path_files -g "$pat" || _files -g '*.(#i)p[bgp]m'