This guide includes some steps which will modify system behavior. I'm not responsible for any consequences. Do it in your own risk. Take Nandroid Backup Before trying this.
1. Redmi 3S Prime with unlocked bootloader (You can also use Redmi 3S if it has fingerprint sensor )
2. Rooted ROM
3. Good File Explorer to Browse Root Files. I recommend ES File Explorer.
4. Good Text Editor Which can edit root files. Try QuickEdit for this.
1. Goto /system/usr/keylayout folder
2. Find a file called Generic.kl
3. Open Generic.kl with text editor (QuickEdit)
4. Search for key 353
5. If you find any line starts with key 353, comment it with # i.e. add # symbol before that line
6. Add a new line with following
key 353 CAMERA
7. Save the file and Reboot.
8. Enjoy Fingerprint Selfie...
Some users may experienced unwanted clicks or touches in screen when they touch fingerprint sensor while the phone is unlocked. That was caused by "key 353 DPAD_CENTER" line in Generic.kl. Here we used that for assigning that key for CAMERA (which will trigger the camera and take picture).
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Note : I'm tested this in Redmi 3S Prime 3GB with Lineage ROM and its working perfectly without any problem.