V2RayNG is probably one of the easier client to use, it requires sideloading.
On the main interface, click the + icon, and Import from QRcode
On your 3x-UI inbounds, click the + icon on your newly created endpoint to expand it, the click the QR Code icon. Use your camera to scan it.
If you skipped the external proxy step in 3X-UI
- click the triple circle icon next to your imported endpoint then Edit