Why do my iPhone x restart by itself every 3 minutes?

That an indication of a hardware failure. You need to have the iPhone diagnosed by Apple or an Apple Authorized Service Provider.

Genius Bar Reservation and Apple Support Options - Apple

https://getsupport.apple.com/repair-locations

If your iOS or iPadOS device restarts or displays the Apple logo or a spinning gear unexpectedly, read this support article for how to resolve this --> --> If your iOS or iPadOS device restarts or displays the Apple logo or a spinning gear unexpectedly - Apple Support