While we don’t know exactly what this update addresses, we can tell you that it downloaded quickly and installed within several minutes. There was no prolonged wait with this update.
Sources say that this third update was necessary ahead of the April fourth update. Update four will add additional features as well as more improvements, bug fixes, and overall enhancements to the user experience. As is pretty much everything Nintendo, we don’t yet know much more than that about the fourth update, so we will have to just wait and see what April brings.