The 3ds has no official GBA hardware support so you are left with emulators. I am not sure where homebrew stuff got to but that many are still using the virtual console says most of what I care to know there.
DS homebrew which normal DS flash carts run does have some attempts at GBA emulators but they are more proof of concept than things you will want to use every day, even the games that do run... I might be biased as I have had GBA flash carts for the better part of 16 years at this point but even so I am used to some sketchy emulation and... nah.
Enhanced DS flash carts can have their own onboard processors with emulators. There are three main ones here called the iplayer, the ismart mm and the Supercard DSTwo family. Though for all practical purposes you will pick the supercard DSTwo or the supercard DSTwo+/supercard 4 in 1 (which is basically a DSTwo but with some 3ds abilities and still sort of available where the straight DSTwo has been had to find for several years now).
For the sake of a forum searcher or mildly interested in history. The iplayer was a very cheap homebrew focused thing technically from supercard but... call it an open secret. The ismart MM was ismart's (they were based around a shop I have forgotten the name of right now, they bought in flash cart designs and tweaked the software for them or made their own) take on the iplayer but with the supercard kernel ported to it by them (and supercard locked down what they could do, hence the more limited emulators available for it, the owner of the site then died and with it the development ceased*) and the DSTwo which is supercard's love letter to the DS flash cart world.
*at one point they came up cheap on the site his employees made after that. At that point they represented really nice flash carts as far as features go if you only wanted one for a DS/DS lite (the DSi and 3ds family were busy being locked down against flash carts with every kernel update and things had not been cracked yet) as not many games had hit after that (we had not quite got
https://gbatemp.net/threads/ap-patch-preservation.477536/ sorted so there were a handful that came out after the last kernel) but all the features were there.