I seriously cannot understand why there are three different subreddits for asking historical questions. And as a potentially-uninformed gripe, the difference between /r/askhistorians and /r/askhistory seems pedantic at best. What is the purpose for having the latter two?
I'll just chip in that there isn't anyone organizing subreddits: if someone wants to create a subreddit, they can just go ahead and do it regardless of whether similar subs already exist. You'll often find several subreddits on the same topic e.g. for a sports team. Over time, the critical mass may all migrate to one sub and leave the others to wither away, but in some cases (like these ones), each sub fulfills its own niche and they continue to live side by side.
My take on the 3 subs:
/r/history is a discussion forum
/r/askhistory is a question forum, and is open for pretty much anyone to answer, something like /r/AskReddit for history questions
/r/AskHistorians is also a question forum, but is meant to be a place where people can ask questions of actual historians. So it's an expert sub like /r/AskScience, /r/AskSocialScience, and several others. As such, there are higher expectations regarding What it means to post a good answer in /r/AskHistorians. As a result, you tend to see a lot of [deleted] comments in here because the mods remove things that don't meet sub standards
/r/history is a general history subreddit. It's not just questions, it's articles of interest. This is appropriate for people asking poll questions, throughout history, and trivia (like "Who is the best general throughout history?"). It's also where you can submit articles and get karma for it (like the constant "this day in history" submissions).
/r/askhistory is actually older than /r/AskHistorians. Our beloved founder didn't know that /r/askhistory existed when /r/AskHistorians was founded. You can be more silly there; it's basically AskHistorians without all of the quality control rules. So you can joke, give anecdotes, and the like.
And, well, /r/AskHistorians is /r/AskHistorians. We in /r/history send people here to get educated and decent answers to specific questions.
essentially you're correct and as a result in practice there are only two history subs: /r/askhistorians and /r/history. /r/askhistory doesn't really exist in practice as a viable option given how few people actually use it (go look at the number of subscribers/currently on the sub and compare it to r/askhistorians).
hypothetically /r/askhistory could be a nice subsection/addition to /r/history and /r/askhistorians for lay people interested in subjects to make historical comparisons and recommend books while in practice that sort of thing just gets separated into askhistorians or history. While there are a good number of deleted comments at r/askhistorians i would also argue they keep the stuff that would be high quality stuff that most accurately would be at /r/askhistory instead of askhistorians and thus help crowd out the development of a real ask history niche.