I also have an N3DS. To quote the tutorial in
here,
"First go
here and download the package where it says
Available here [1]. Inside is the rxTools version you need. Now google for Reisyukaku Pastebin and download the ReiNAND CFW you find there. Inside will be a firmware.bin, which you rename to 0004013820000002orig.bin, then you create a subfolder \data in the rxtools folder and put the firmware file in there. It has to look like this at the end: microSD:\rxTools\data\0004013820000002orig.bin"
After that, try launching RxTools.3dsx from HBL (or CTRBootManager if you want to set that up)
From my own experience, I think this build has a 90% chance of successfully booting up, with a 10% chance of crashing. After that, update (not downgrade) with the original sysupdater by profi200 using the 9.5 pack for your N3DS' proper region.
You can find the pack on the iso site.