So yeah, Undertale on a 3DS console is something I have been thinking about a lot but it really isn't possible. But what is possible is to make a version for Gameboy which just goes up to half-way through the ruins and use NSUI to make into a vitual console CIA. that is exactly what this is. You will need to have CFW on the 3DS Console you want to download it, you should also have the FBI app downloaded too (you should get it when you first installed CFW). I'm not going to go to in-depth on how to download CIAs put you can find a tutorial online really easily.

Credits to Mr Chronomaniac for making the rom