Basically yes to most of that/"if you are just planning to use it on a DS then carry on".
To go a bit more in depth
The GBA requires fast access memory to run and NAND memory (which is most of what you would have seen in USB drives, SD cards, CF cards and everything else is too slow). However NOR memory and various types of RAM are fast enough for it.
NOR memory is very slow to write, expensive for lots of space and has a lot of issues that prevent it from being entirely user friendly.
RAM tends not to maintain data when you lose power and also quite pricey when it comes to space.
Early GBA carts (everything before the supercard and EZ3) attempted to make increasingly larger collections of NOR memory.
Later GBA carts stored things on NAND and copied it to RAM (or in some very odd cases NOR) allowing them to vastly increase the amount of games they could hold at the cost of writing a game to another sort of memory. Most of the later cards has a NOR section as well so people could store their favourite games and not have to wait the 20 odd seconds to copy to RAM.
The 3 in 1 is an expansion pack which various teams cooked up when they realised the DS slot to GBA slot transfer is not that slow and if people already had a DS cart which could store loads of GBA games (and given SDHC which starts at 4 gigs exists and GBA games tended not to go above 16 megs that is a lot) they might as well jettison the NAND memory and drop the price.
The 3 in 1 had its software read/write stuff open sourced and so everybody supported it either at cart level, with the help of another program or some combination of the two.
Best... they tended not to have RTC but for the most part as they are running on an actual GBA using something that was fast enough to work then yeah it will play damn near everything much like a normal GBA cart. The downside is that you need a DS to manage it unless you want to write one game to the NOR and have it load from that on a regular GBA. The DS has no link port for the GBA so if you want to do multiplayer there then you will need one, this then means you have to write to the NOR on the DS, swap over to the GBA and then play if you want to use it in a GBA or GB player or something that is not a DS/DS lite.
If you are just using it on a DS then the worst thing you have to do is manage the saves and most programs will do that for you anyway. Speaking of saves and link ports you can also try emulating the games that want linking.
