I'm not sure about flashcarts for the 3DS - I've never looked into them. If you jailbreak your 3DS using the guide at https://3ds.hacks.guide, you can run homebrew directly from an SD card.

If you decide to go the flashcart route, please post your impressions here, I'm curious to hear how they work.

3DS Hacks Guide

A complete guide to 3DS (and 2DS) custom firmware, from stock to boot9strap.

^{Nintendo Homebrew (3DS Hacks Guide)}