I'm in the middle of the second three-book arc of what was once a three-book series (so, this is books 4-6). When I wrote the first three books, I wrote them all at once before committing to them to Amazon. And now as before, I'm writing all three novels at once before releasing anything.
I know it's tempting to get your work out in front of an audience as soon as possible, but to me, when I'm writing serious fiction and not serialized comedy (which is a whole different kettle of fish), I think it's an advantage if you can put the entire series to pen before releasing it.
You can go back and change things if your narrative demands, add foreshadowing for things you hadn't thought of yet, and so on. You can catch problems and fix them before anyone sees them. And when it's time to release, you can put the whole series out at once so people don't forget about them.
What do you all think?