Camilla, Kate and Charlene are now very much royals, by marriage.
The protocol is actually pretty simple (for most European countries):
- Camilla has to curtsey to all those who are of higher rank than she is. That includes all Monarchs and their spouses (such as King Harald and Queen Sonja of Norway), as well as Heirs Apparent to the Throne (such as Crown Princess Victoria or Prince Felipe). She doesn't have to curtsey to wives of other Crown Princess (such as Crown Princess Maxima) because they are her equals. Everyone else has to curtsey to Camilla.
- Kate has to curtsey to all those who are of higher rank. That includes all Monarchs and their spouses (such as Prince Albert and Charlene), all Crown Princes and their spouses (such as Crown Prince Haakon and Crown Princess Mette-Marit), all younger sons of a Monarch and their spouses (such as Prince Laurent and Princess Claire of Belgium), the Monarch's daughters (such as Princess Anne), and sometimes the Monarch's grandsons (but not their wives).
- Charlene only has to curtsey to Monarchs (such as Queen Elizabeth) but not their spouses (such as Prince Philip) who are her equals.
I am completely at a loss why would Charlene curtsey to Charles and Camilla, why Camilla wouldn't curtsey to Charlene and Albert, but would curtsey to Crown Princesses of Romania and Thailand, and why Kate curtseyed to the Duke of Gloucester. Basically, as someone mentioned in this thread, those rules aren't carved in stone and royals have relative freedom of choice. Mostly, they are guided by common sense and respect; it would be quite preposterous, for example, to expect Kate to curtsey to Prince Christian of Denmark should they meet antimony soon, although the boy does outrank Kate at this point.
Charlene chose to curtsey to Charles and Camilla out of respect, same as Kate and Duke of Gloucester, or Camilla and Crown Princess of Thailand.
Alternatively, there might have been so many royals around that female royals got a bit confused and just decided to curtsey to everyone to be on the safe side.