Black screen and bothersome voiceover, iPhone SE

This problem has occurred t three, well now, four times. If I touch my phone screen with three fingers, the screen will go blank and all I will hear is a very annoying VoiceOver talking very fast. I managed to get the password screen to come up but it does nothing. I really don’t want to go to the T-Mobile store where I bought the phone again. Does anyone know how to correct this problem and I have never heard of anyone else ever having this problem.

oh, this phone is fairly new