Hm... If I understood correctly, they first adapted the first part, then the last, then the second. That happens sometimes and it's understandable from the sales POV (the first and the last parts are often the most interesting etc.), but it's frustrating for viewers who then try to find all the parts and view them in the logical order
As for series, I don't yet know perfect sources, but here are some decent ones, besides Miss AV (which in this case only lists 2 out of 3):
JAV Guru, JAV Land (seems defunct, but you can use archived pages in Google, Yandex, Wayback Machine), and finally, Duga/Fanza (if you have a Japanese VPN)
What's good about JAV Guru is that they give the original name of the series in the URL, so you can easily cross-check it on Duga/Fanza
So, basically, I would rely mostly on JAV Guru at the moment as the most accessible one. You don't have to search the
series page etc. (actually, it doesn't list all the series there are... frustrating again!), you can just find a title and see if it's a part of any series in its description. If it is, it will literally say so on the page, e.g., in this case:
P.S. Oh, also, there is X-CITY, but their data is also not complete, I believe. Basically, if a site is not just a database but sells titles as well, their database might be limited to only those titles they sell. Which also reminded me of this site, a database that lists series and is still functional: JAV Database. It seems obvious, but I'm not used to using it yet
So, I would use two actually, JAV Guru and JAV Database, maybe with some extra check using Duga/Fanza if you can access those. Anything else I know is less reliable in terms of completeness