I succefully downgraded my o3DS, but I'll wait KTM's release to downgrade my n3DS because I soft-bricked it twice and I don't want to try again with Sysupdater.
Ok, I managed to get it to work, for some reasons you just have to rename your "rxTools.dat" in "Launcher.dat" and launch it with gateway: http://go.gateway-3ds.com/