After many months of fans pleading to bring Miiverse functionality to the 3DS, Nintendo has finally complied. After installing the latest Nintendo 3DS update you will not only be able to access the Miiverse, but will also be able to link your 3DS to your Nintendo Network ID. This will allow you to share system activity and, more importantly, funds across both systems.
Unfortunately, this isn’t a true account based system, as you cannot sign on to any 3DS and access your digital games. In addition, you can only link your Club Nintnedo account to one Network ID, which can only be linked to one 3DS. You can transfer games between 3DSes, but you need both 3DSes there to be able to do so, and doing so deletes them from the 3DS you are transferring from. Once again, it doesn’t do much for people with broken 3DSes that buy a replacement.
The 3DS version of Miiverse is still a little cludgy. For example, you cannot add new friends on the 3DS. Instead you have to friend them on the Wii U and this will trickle down to the 3DS. In addition, 3DS online games will still require the use of friend codes, which was the primary reason users wanted Miiverse functionality in the first place. Nintendo, you still have more than a bit of work to do to get this working right.
Source: Nintendo 3DS Update