To anyone wanting to play a custom map that isn't on this forum yet (i.e Nightmare), you can easily download most steam workshops that will result in the same file you get from using the links on this forum at this link:
. Steam is still not required. Simply find the workshop using google, copy the URL into steamworkshop downloader. When the file is downloaded, you simply follow the same steps to get it into bo3. This also works with mods.
I have found this to actually download a bit faster than MEGA and you won't have to worry about any download limitations the same way you would with the free version of MEGA. If some of the custom maps you download give you an error message when you try to play them, make sure you are using version 100 of cracked bo3 (not 88).