1)
Will he judge only Aniyim?!
1.
Radak: He will judge their case against those who afflict them. This is like "Shafteni Elokim v'Rivah Rivi" (Tehilim 43:1).
2.
Malbim: Not only will he judge the rich. He will judge also between one Oni and another; a case about one Perutah is like a case of 10,000 Dinarim.
2)
Why does the verse mention these three matters?
1.
Malbim: The king's judgment extends over three matters. (a) Mishpat Bein Adam l'Chavero regarding Kinyanim, loans, etc. 1 (b) To give to the Evyon, who lacks food, his needs. (c) Mishpat against thieves, extortionists and oppressors.