Maxima was created Princess of The Netherlands, Princess of Orange-Nassau, Royal Highness, in her own right, and she is Princess of Orange by virtue of her marriage;
Laurentien was created Princess Laurentien of The Netherlands, Princess of Orange-Nassau, Royal Highness, in her own right;
Mabel is Princess of Orange-Nassau, Royal Highness, by virtue of her marriage.
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This may be reaching but based upon my personal research into royal/imperial history, as it applies to present-day members of varied royal houses, I'd have to question the source of your information. According to the Almanach de Gotha (which is the perennial authority on "blue-bloods"), the former Máxima Zorreguieta acquired the predicate "Princess of the Netherlands, Princess of Orange-Nassau" with the title "Royal Highness" which entered into effect upon marriage. That said, she has and only ever will be (at least until the ascension of her husband), Her Royal Highness Princess Máxima of the Netherlands. She is not "The Princess of Orange" despite being married to "The Prince of Orange", or for that matter "Crown Princess Máxima of the Netherlands" (a title which has never existed there). That title is officially restricted to only the Heir/Heiress Apparent (which explains why neither Queen Beatrix, nor her mother or grandmother, ever held it!!).
That said, it can be safely presumed that at no given time will there be a "Prince and Princess of Orange" (which would suggest dual heirs apparent). However, in my opinion, it would be far more convenient (in language and print) to use "The Prince and Princess of Orange" rather than "The Prince of Orange and Princess Máxima of the Netherlands", which takes up quite a great deal of space and effort.
The former Laurentien Brinkhorst acquired the same titles as her more famous sister-in-law, however only as a courtesy (by virtue of her marriage to a man who holds the titles). She is, through marriage only, allowed to be referred to as Her Royal Highness Princess Laurentien of the Netherlands.
You are quite correct though in your status appraisal of the third sister-in-law, who by virtue of her marriage (which lacks governmental approval) received the courtesy style Her Royal Highness Princess Mabel of Orange-Nassau (never of the Netherlands, as her senior in-laws).
When you think about it, they're (the Dutch) honorifics seems to parallel the British in that upon marriage to a royal prince, the bride takes her husband's most senior title (i.e. the former Lady Diana Spencer upon marriage became Her Royal Highness The Princess of Wales, the feminine equivalent of her husband's most senior title - though NEVER PRINCESS DIANA, as ALL her royal titles were courtesy of her marriage).
This is also the reason why Laurentien and Margriet were already there when the carriage that carried Beatrix, Maxima and Willem Alexander arrived 
