And the 'three or more' thing is just plain wrong...Sometimes it is five or more (or even higher than that) ... doesn't hold because the three comes from there needing to be the named and others.
One probably wouldn't use it for three people, but it wouldn't be wrong.
What does that mean, @Pedant5corner ?
Do you mean that three have to be named before you state 'et al.'? Again, that depends on the publisher, and also on where the citation is occurring (for example, within the text or in a reference list). It also sometimes depends on how many authors there are).
The point being that these are just highly variable style decisions made for specific publications. They don't translate into any kind of universal requirement for correct language. A style guide may well forbid any informal use of et al. (partly for reasons of tone and partly because it would be potentially confusing alongside its use in citations), but there is no general rule forbidding this.