I reported it on the Feedback app. My iPhone isn't responding, it's completely shutting down, booting to Apple logo, shutting down, booting to Apple logo, and repeating a couple times, and then completely booting up. I have not noticed my iPhone get at all warm while charging.
Also, I tired doing a fresh install of the iOS 11.3 beta 3 IPSW, and it still did it. So I'm guessing it's beta 3 that's the problem.
Also, it only started happening in the last two days and it has not been happening over the whole beta 3 period.