Cron job to delete files older than 3 days

This is easy enough (although note that this goes by a modification time more than 3 days ago since a creation time is only available on certain filesystems with special tools):

find /a/b/c/1 /a/b/c/2 -type f -mtime +3 #-delete

Remove the # before the -delete once you are sure that it is finding the files you want to remove.

To have it run by cron, I would probably just create an executable script (add a shebang - #!bin/sh to the top line of the file and make executable with chmod a+x), then put it in an appropriate cron directory like /etc/cron.daily or /etc/cron.weekly. Provided of course that you do not need a more specific schedule and that these directories exist on your distro.

Update

As noted below, the -delete option for find isn't very portable. A POSIX compatible approach would be:

find /a/b/c/1 /a/b/c/2 -type f -mtime +3 #-exec rm {} +

Again remove the # when you are sure you have the right files.

Update2

To quote from Stéphane Chazelas comment below:

Note that -exec rm {} + has race condition vulnerabilities which -delete (where available) doesn't have. So don't use it on directories that are writeable by others. Some finds also have a -execdir that mitigates against those vulnerabilities.

You would be much better off using tmpwatch

tmpwatch recursively removes files which haven't been accessed for a given time. Normally, it's used to clean up directories which are used for temporary holding space such as /tmp.