First, a primer on Atelier announcements, as far back as I tracked down. I won't say the announce dates are 100% precise but they should generally be pretty close.
|Mana Khemia 2||2008-05-29||2008-02-13|
|Escha & Logy||2013-06-27||2013-03-25|
|Shallie||2014-07-17||2014-03-24||Initially 2014-06-24, delayed 1 month|
|Sophie||2015-11-19||2015-06-22||Initially 2015-09-25, delayed 2 months|
|Firis||2016-11-02||2016-05-23||Initially 2016-09-29, delayed 1 month|
|Lydie & Suelle||2017-12-21||2017-06-06|
So what can we learn? From Lilie to Escha & Logy the series was released in May or June on a yearly pace. Shallie distrupted this with a delay of one month, and Mysterious slid the schedule back harder, then we came to Lulua and Ryza which just didn't care about timing at all apparently. For many years it wasn't unusual to announce games (usually via Dengeki) a few months ahead of release. This seemed to widen with Firis and now seems to be four to six months out.
Now I'm just going to speculate for a minute, please do not take my word for anything said in this paragraph. My guess for the Ryza 3 announcement is that it'll happen within a month, probably before Tokyo Game Show (end of September). They aren't as reliably yearly as they used to be, but TGS is a major promotion time so I'd expect them to push hard for it. It furthermore would not be surprising to see a January to March release window for the next game, looking at the gap between Ryza 1 and Ryza 2.
Now watch Koei Tecmo prove me wrong.