Hello! The IBM PT-3, or IBM PT-3 Service System (it's full name) is a computer that seems to have been created solely for diagnosing other IBM machines (most likely large mainframes). It was created in the late 80's or early 90's to replace the IBM PT-2, the previous generation of IBM service system. It was a tool only for trained IBM engineers and was never released to the general public, as far as I know. I'm currently performing some research about the history of this system and what kind of mainframes/systems it was meant to diagnose and interact with, and am hoping to prepare a blog post to publish on it soon.
I know it's been 7 years, but hopefully that provides some useful background information! Funnily enough, your post is the only relevant result on all of Google if you search IBM PT-3!
