@barnee
Yes, the third Army in the same terrirtory is formed - but is not active, since only 1 Army can be active in the same territory.
And yes - the units inside this third Army counts towards the total of 10 landbased combat units that is needed to form an Army Group.
Remember:
Multiple Armies and Army Groups can be formed and share the same territory.
Example:
Two German Army Groups share the same territory: Western Germany. During a German attack, 1 Army Group attacks US forces in France - and 1 Army Group attacks British forces in Holland/Belgium.
Both Army Groups attack as Army Groups - and each Army Group get the 2-dice bonus rolls at “4” - since they no longer share the same territory.