Thierry the First, son of Clovis, was king of Metz and Orléans (Metz could arguablly be related to Austrasia, but Orléans would not with Neustria as the kingdom of Paris have quite better claims to be so).
Thierry the Second, son of Childebert II, was king of Austrasia and Burgundy,
Thierry the Third, son of Clovis II, was king of all Franks.
Thierry the Fourth, son of Dagobert III, was king of all Franks
So, Thierry, son of Childéric III, could only be the third king of Neustria/West Francia.
The game is not able to keep a same numbering for distinct kingdoms (which might be useful for Spanish realms). Anyway, numbering kings is more a things of historians. No one would say Thierry the Fith when talking about or talking to the current king. One would simply say Thierry.