It would be fair to open the 3rd Manufactory slot in the Drone Age, and the 4th slot in the Auto Age, but at the same time, the Auto Age must necessarily get something that is really not available in any other Ages. This will be the compromise solution.
It is logical to assume that if this is the age of automation, then robots should appear to make somehow the work of citizens easier or improve other processes.
For example, a House-angar, where not citizens live, but robots who can perform only construction work (improve buildings), but not study at Universitet. Yes, I know that there is already a Wonder of the World "Burj Al Arab", which increases the productivity of citizens, but let robots increase productivity of citizens even more or something like that.
Or let robots somehow increase the productivity of Barracks, Factories and Airfields for restoring troops.
It depends only on how much imagination of BHG can afford, and the abilities of the development team.
In any case, in the Auto Age there must be some breakthrough technologies.
P.S. Probably, as always, I am talking about what I would like to see in the game, but it is unlikely to be implemented.