It seems like brother 1 and 3 are finding something that is not necessarily an opposite (foil) to what they set out to find...wisdom/beauty, heroism/love... while brother 2 looks for justice and finds ??? (compassion/mercy). I would suppose that each of the brothers thinks that what they are looking for is the "primary" trait of a good king...and finds that the opposite trait is just as important (if not more so). Finding a trait that is not an opposite seems to me to be a little random...based solely on my vast knowledge of the rest of your story, of course
For symmetry, each of these brothers should go through the same type of process to learn...be presented with a problem/situation where they choose the trait they are looking for as the solution, and find through trials and tribulations that their initial trait choice will not always be the right answer.
For Justice/Compassion(Mercy) the thief could turn out to be a friend who is doing something illegal but for the right reasons. The prince chooses "following the law" as his course...goes undercover to track down the bad guys...eventually finds out that the theft is ultimately for a really good cause - to help someone desperately in need - and then finds out who the thief is.
-OR-
The prince finds someone (some group) on his travels that he really desires to help - but (for some reason) he can't call upon his wealth/nobleness(?) to solve the situation - in fact he can't help at all because the laws and situation are set up to make anything he tries illegal.
The discovery that not all laws are just, or fair, in all cases...and that a good leader needs to know when not to be rigid and when to be compassionate... and when to get the law changed...are a great lesson!
FWIW I think a good king would have to have ALL of the traits - so I like the fact that the other two princes are advisers. ...but please tell me that "beauty" is not skin deep
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