Sync between PC’s

At my home network I have mostly 3 PC’s. The main one I use has lotsofMP3’s and Movies on them. Every so often I manually copy the MP3’s or movies to another PC so I have a backup. I am looking for a easier way to do this. Prossbably having the backup PC sync with the main PCevery night. Could this be done within Windows or some 3rd party app?