I do not have a PS3 but I guess it is dlna compliant. Thus, if you enable the UPnP AV server in the DNS-320, you should be able to see it.
This can be done by clicking the management tab, then Application management, UPnP AV Server, and you have to select the folder you want to share. Once it is activated, you will see the DNS-320 as a media server.
Depending on the amount of files you want to share, the web UI may take a time to refresh the list. In my experience, sometimes it takes a long time to refresh but it also happened that it took less than a minute.
Good luck.