But thanks to you, I now know. Three things and that's it. Roughly speaking, of course. But nevertheless, a hard number, three.
No, a rule of thumb is not a hard answer. A rule of thumb means 'start here and adjust to fit.' A good rule of thumb rarely needs much adjusting, but the option is still there and that is why it is a 'rule of thumb' and not a 'rule.' I personally find that, for the purposes of introducing a character, three details fits very well with little adjusting. You might find that four fits better, and that's fine. You might even find that ten fits your style better; I may not agree, but then writing style is a personal thing.
It also varies with the situation; my WIP starts with a deluge of description - but then the opening scene is a child 'playing pretend' with all the detail of her imaginary world actively in her attention. When most people talk to someone they know already, they don't usually have the looks of the person they're talking to foremost in their minds; the point I was trying to make in this article is that choosing what a person notices about another tells a lot about the person doing the watching - the '3 items' rule of thumb was incidental.