Why not torrent PC game updates?

Some companies like Blizzard use torrents for downloading updates (though you can disable it in the launcher settings IIRC).

The hurdle is that need to verify that each file isn't modified. Otherwise someone could spread game hacks by modifying the game files and sharing them. E.g. make other players the drop all their items or what else the modified client does..

The simplest solution would be to use a single singed file per torrent. However, that way users need to keep a copy for sharing after extracting the contents. In your example ~3GB occupied on the user's hard-disk space in order to safe you some traffic.

The other thing to consider is esp. for F2P you lose potential customers if the download takes too long (and users abort). So unless you are sure that enough users are sharing I wouldn't recommend it.