Unity for 3DS is proprietary software, along with the official SDK which it relies on to build projects. You will first need to sign up to the
official developer portal to claim your neccessary tools and licenses to be able to start developing.
With Unity, you will need to first get used to a lot of stuff within the editor (it took me ~2 weeks to properly get used to nearly all of Unity's features), alongside plenty of C# and/or JavaScript knowledge for scripting components.
If you're new to development, be sure to read up, and watch, many in-depth tutorials to aid you along the way. Unity is packed with hundreds of features and script functions to learn, and remembering all the neccessary features may not be easy.
But I digress. You might be a quick learner; you might not. You might even already know a thing or two about Unity!
All comes down to experience at the end of the day.
As for 'bringing a Unity game to the 3ds', the only option you pretty much have in terms of "legal distribution", as an individual, is submitting to the eShop. You may be putting yourself at risk if you distribute your project elsewhere outside of the official development sites; into the public's grasp. But that's another story...
If you feel that you have fully completed your project and is ready to be made public, send it over to LotCheck (the submission process), where they will evaluate your game/app to see if it is deemed suitable enough for a release on the eShop.
There are a lot of LotCheck requirements, rules, and regulations, but it's too in-depth to explain further here...
That's pretty much all the basics of development and publishing for a Unity game in a nutshell. Feel free to ask any more questions you may have and I'll try to answer them the best I can. And even though I have never had a Unity game of my own published, I know quite a bit about Unity and its features.
Oh, and if there is any more info I have missed out here, someone feel free to fill me in!