A Nintendo Lawyer has clearly described what the company may or may not deem illegal regarding the emulation of their ...
The developers behind the upcoming Azahar 3DS emulator have laid out their argument for legality in a blog post. This includes several changes to the Citra formula to stay on Nintendo’s good side.
You can use an emulator like Delta to play games from the NES to the DS, and Retroarch to relive some OG PlayStation titles.