The first time I encountered the concept of a 3xer was in the context of political radicalization, people infusing their mind with YouTube self-radicalization content on 3x (or higher) every day. My mind conjured up images of Malcolm McDowell in that A Clockwork Orange scene, only that it's self-inflicted and with content aiming at the exact opposite.
However, a lot of TV shows - particularly USA ones seem to needlessly repeat everything like there running a lecture for amnesiacs. Here's what we're going to say in the first part, here's the first part, we say what we said we would, now a recap, then a break so we review the whole first part ... now we're 10 minutes into the show and we've seen about two minutes of unique footage. It's harrowing -- I'll take overdrawn explanations in preference to that.
Closer personal example: I spent weeks trying to bully a supervised machine learning approach into a reinforcement learning one, because the 800-page reference book I used (that claims to cover all machine learning, and is well regarded!) in no way acknowledges the existence of this sub field. For whatever reason, and across multiple fields, I've never found static text to be good at "here's what you should be looking for", and I don't think it's reasonable to discount that knowledge as being valuable.
Nearly all very-large-texts I've read on technical subjects are poorly written. The early and later sections seem to have little relation to each other; Some parts will be too general and other parts too vague. It's like the author totally loses perspective.
There's a sweet spot of around 250 A5 pages where a subject can maintain consistent scope and have meaningful relationships between chapters.
There are exceptions, but they are few.