Considering the success that Bethesda Softworks had with Wolfenstein II, you had to know they wouldn't pass up the chance to do a trilogy. In an interview with Metro, Bethesda Vice President Pete Hines left absolutely zero doubt on the table that a third installment was in the works, but have little detail as to any of the details as the question asked has a short response.
GC: Ultimately I just want you to guarantee there'll be a Wolfenstein III, because I want to see how it ends.
PH: Yeah, sure. Absolutely we're making a Wolfenstein III. They said on stage that they're taking a break from the larger story to do this thing. But we all have to see how that ends. [laughs]
Considering the timetable for the previous two games, and the fact that Bethesda has given no word about it so far, we're guessing the earliest you'll see anything from it will be during the next Bethesda presentation at E3 2019. Trust us, unless someone scores the leak of the year from the company, that's when you'll learn more.