Leave your o3DS and o2DS alone. Their parts are not cross compatible with the n3DS, and it would be pointless risk damaging them.
Do the hardware check after you hack the n3DS, backup the SysNAND *.bin image, and flash it with CTRTransfer.
You want to first exhaust software options before attempting hardware debugging.
If your CFW install goes well, in an application called GodMode9 you can safely test several hardware components.
- Various button responses. Some of them will not be I/O observable.
- LCDs and speakers are already verified good..
- Sleep switch, SD card reader, and game cart slot reader are good if you successfully hack with ntrboot.
- NAND check if you can boot Luma3DS or GodMode9 on CTRNAND (not from SD card).
That just leaves out IR port, cameras, touchscreen digitizer, 3D slider switch, c-stick, microphone, etc...