Well, you'll need a Slot 1 Flashcart to play GBA games on it.
gbata is required to patch GBA ROMs, or you won't be able to save.
Then you use Rudolphs Tools to flash GBA games on it.
You have to put the gba games on your card.
You first patch the gba games with gbata (downloadable form this site).
Then you put the .gba files and rudolph's tool on your R4.
Open up rudolph's tool on your R4 to copy the gba file onto the 3-in-1's memory.
The 3-in-1 also works with the DSTT (top toy). I just got both, it's pretty easy. I haven't put any .gba games on the 3-in-1 yet but got the DS Opera Browser to work (after patching), so the "memory expansion" part of it works great and only took a minute to set up.
I'd really recommend taking a look at the 3 in 1 wiki. Thaigrocer and I put alot of this information together, and it addresses initial set up and a few questions you may see on the way.
I'll defined gender lines is comedy gold 🥰 not sure laughing about it... Does that make me insensitive or just accepting and it's funny that people get angry about trans stuff? Lol