Please think of fairytales... What word would you use when you refer to a prince's clothes? Is it a dress?
If you want a term to describe all of a prince's clothing, I would use finery.
Expensive or ostentatious clothes or decoration: officers in their blue, gold, and scarlet finery http://www.oxforddictionaries.com/definition/english/finery
But if you're looking for the 'dress' part of a fairytale prince's clothes, I think it would be tunic.
A loose garment, typically sleeveless and reaching to the knees, as worn in ancient Greece and Rome.
the special clothes that are worn or objects that are carried at official ceremonies - oxforddictionaries.com
"Raiment" is often used in this context. "Tunic" refers to a particular garment; "raiment" is more general.
"Garb" or "dress" (without an article) is a rather drab abstract noun for what a person is wearing. One does not refer to "a garb". "A dress" (or using "the" to refer to a particular "garment") means an item of clothing that is normally worn by women. Thus, "a dress" is inappropriate for a (male) prince, unless he is pretending to be a woman.
If I was writing a story I would refer to the Prince's costume.
If you're looking for a catch-all for what the prince is wearing, consider attire. The prince's attire was befitting a man of his station.
If you're trying to find what clothes the prince was wearing, that would be dependent on where the prince was and what his local region wore.