[Android Central Question]

I registered my left index finger as an ID but when I use any other finger after touching the screen 3 times the phone opens

[B. Diddy]

Welcome to Android Central! Fingerprint sensors are not entirely secure (which is why if you use your phone on a company account, the security policy often prevents you from using biometrics to log in). The system isn't storing your entire fingerprint -- it's only storing a relatively small percentage of the data points. For the most part, it should only respond to your fingerprint, but it can be fooled if someone else whose fingerprint matches enough of those data points tries to unlock it. (I have a friend in the cybersecurity business, and what he tells me about how insecure most systems are is pretty chilling.)