I guess the three orange apples possessed a special healing ability as described by the doctor. So I'm wondering now if any young man finally gets the orange apples to the princess will the doctor still have his gold? as he is not the one that healed her.
I think the central theme of this episode is sincerity. The eldest son of the three boys did a nice job by being courageous and brave enough to travel all the length to get the three orange apples. But his act of not being truthful made it turn into a toad. Well, he was probably trying to be smart not to give the old woman part of his oranges.
The second son is off to the adventure now, hope he learns from his brother's encounter.

You guys are doing a nice job in the vineyard. Well done sir.