For Linux files you can download the .deb files for the packages you are interested in from the
Launchpad repository and extract them. Debian packages are just ar archives with a
specified structure. The files for each program will be inside the data.tar.gz for that .deb file.
For Mac, the files you're interested in are in a file called Archive.pax.gz inside the .pkg file. You can extract the files you're interested in with gunzip and
pax.
For Windows,
it looks doable but I can't test it because I don't have any Windows machines nearby.