Why is my iPhone se powering off by itself every 3 minutes??!!!

There could be an issue with degraded battery health or it could be that the power button is stuck pressed down which forces the iPhone SE to reboot.

For battery health, if your phone doesn’t reboot quickly enough, does going into Settings > Battery > Battery Health show any alerts?

Probably because it needs a new battery. Go to Settings/Battery/Battery Health and see what it says.

Just to be clear, by powering off you don’t just mean that it goes to sleep and can be waked by pressing the side button. You actually have to hold the Side button until an Apple logo appears, which is the definition of powering off.