There is a site from some darth guy. sternie I think is called, you can download all 3ds firmware a search engine will be your friend.
By example, the latest firmware for my New 3DS (11.17.0-50U) have a MD5 Hash of: 6e81f4fe7b892fc89c2f4be604731475.
You download that archive containing all .cia files.
Theorically you can use NUS to download all cia files too.
Then you can use GodMode or
https://github.com/profi200/sysUpdater to offline update your console.