I originally wrote this essay in April 2018. Now, just shy of three years later, I see there's a need to clarify its intent.
When I was active at AFD, I would often find myself looking at a long reference list, most of which were crap. My first instinct was to say, "Most of the references are crap, so the article must be crap too". That clearly wasn't fair, but the alternative was to slog through all the junk to find the few gems. That was too much work, so I started asking the article authors to point me to the best sources.
After a while of writing the same request over and over, I put on my software engineer hat and refactored. The result was this essay. I distilled my most frequent comments, wrote them down, and pointed people to it. I soon tired of writing User:RoySmith/Three best sources over and over, so I made the WP:THREE shortcut.
To my surprise, this became a popular essay. As of 28 March 2022, there were 2171 incoming links to it. Unfortunately, many people misunderstood the essay. People started treating this as if it were policy, which it's not. It's just my personal essay. Some people went so far as to modify it. I mostly pushed back on this. Yes, I know about WP:OWN, but seriously folks, I deliberately put it in my userspace, and wrote it in the first person. WP:USERESSAY is also a thing.
Lately, I've come to realize that there's an even deeper misunderstanding here. This was never intended to set a standard that three good sources is either a necessary or sufficient condition for a topic to be considered notable. It was just a way for me to more efficiently find the best references to zero in on during my evaluation. People are still welcome to link to WP:THREE, but if you do so, please use it in the way it was intended. Are you looking for help in sorting through a long reference list? Great, I'm happy to have you link to this. Are you trying to argue that a topic needs three sources to be notable? Nope, that's not what this is about. Maybe you're looking for WP:3REFS instead? That's also just an essay, but at least it's one which talks about notability requirements.