People who crank out a book in a series every three months aren't individuals, they're teams of writers under one pseudonym, and they're always given a formula for the plot, they just have to fill in the details. It's the paint by number of writing.
The most an individual author who's genuinely making up their own series can do is one book every year or two.
The fact is everyone is different. There are individual authors out there who can crank out books that fast. And it's not beyond reason that they can. There are lots of selfpublished authors that have to maintain that pace to pay the bills. Fear and the desire to eat is a great motivator to get butt in chair and get the job done.
If we just look at the simple numbers if an author writes 2k words a day (Which is what Stephen King's pace was in his prime) 5 days a week, that's 10k words per week, 40k words a month, so writing a 100k book in three months is easily doable. Also some of the books may be only 50k words, which makes it even more achievable. Also writing more than 2k words in a day is very doable.
I was once listening to a podcast with Michael A. Stackpole. It was NanoWriMo time, and as a demonstration, he decided to do double the 50k word goal. And over the course of that month, he described the process of how he could write 100k words in a month or one book. He's not a titan of the publishing world who can hire a team. He's just a working pro who knows how to put a story together. If you listen to podcasts interviewing the authors who do novelizations of movies, the reason they get hired is their ability to write fast. And that translates to their original works, too.
How people can crank these out every three months is quite beyond me.
Some people are just built to write and write at that pace. There's the story of Asimov I believe, he had multiple typewriters, each with a different story going in it. When he got stuck on or tired of one story, he'd simply shift to another typewriter and jump into that story. I believe he wrote around 500 novels over his life this way. But to be fair, he also stated he had no life.