You can't download Halo 3 files directly from Bungie.net anymore. You'll have to find someone who has the maps in their fileshare or on a separate website and download the file(s) from there.
Here's one way I download Halo 3 usermaps with the help of Bungie.net
1. Find the file you want to download
2. Send a friend request to the person who created/uploaded the file
3. Launch Halo 3 (if you haven't already) and view their file share through their gamercard.
4. Download the file from their fileshare (if it's still there) and tada, you're all set! You can cancel the friend request after this, if you want.
This only works if someone still has the file in their fileshare. You can also check other players' fileshares to see if they have what you are looking for.
Most usermap links on other websites are dead. I have a bunch of maps I may upload, though.