I did a write-up on the 3-in-1 and posted it on GameFAQs since so many people seemed confused about it, and it got deleted for 'illegal activities' (mentioning GBA support?), so I'll repost it here where it may be of some use:
When I bought mine, I had no idea what I was expecting, and it took me a few hours to figure it out because no one really explained it well, so maybe with this (half-asleep) post I can help others. Here's how it works:
You don't really store stuff on the cart. It's more like a really sexy option pak. You store your GBA ROMs on your Slot-1 card (R4, etc), along with Rudolph's tool. You leave the ROMs uncompressed, and you use the GBATA tool to SRAM patch them if possible, or else you can't save.
Then, you boot the tool like any other homebrew application (comes prepatched for R4 DLDI if I recall, might be wrong). Takes you to a menu where, with the L an R buttons, you can choose three modes: PSRAM (default), NORFLASH, and Option Pak.
In PSRAM mode, you choose a ROM in your GBA directory, and if it's 16.00 MB or less (this would be all but a handful of GBA games), it'll load it into the PSRAM in just a few seconds time, then soft-reset into GBA mode.
In NORFLASH mode, instead of loading the ROM into the PSRAM, it will erase the NORFLASH, then write the ROM to NORFLASH. This is a very time-consuming process, and only necessary for the handful of 32MB (256Mb) titles. It does have an advantage; the ROM remains in the NORFLASH after the system is powered off, and can be played on other DS Lites, if you wanted to do that for some reason.
In Option Pak mode (it might be called something else, I'm doing this from memory), you can choose from Expansion Pak (the RAM addition for Opera and some homebrew), Rumble Low, Rumble Medium, and Rumble High. After making a choice here, it soft-resets back to your Slot-1 menu, where you then choose an app that uses the feature you just chose.
Overall, it's a great little product, and the price is just right. As long as you remember to SRAM patch your ROMs beforehand (GBATA works great for this), you shouldn't run into any problems with it. Just remember that you store the ROMs on your Slot-1, PSRAM is faster than NORFLASH, and Rudolph's tool is essential for enjoyment (and proper operation), and you'll be fine.
Hope this helped some people!
