Crush. Mix. Burn. Repeat.
All I am aware of is that they plan a reprint of Marne 1918. Have not yet heard anything about a third game in the series, until checking their side due to your notification:
It is very likely that the next game in the series, simulating the offensives in Flanders and allowing players to play out a huge campaign game including all of the games in the series, will use the advanced rules from Marne 1918. However, we will be very mindful of players’ reactions to the new system and it is possible that the final advanced rules will be a mix of the rules from the two preceding games of the series.
I guess you should just drop them a mail, they will surely answer!