The only thing I can say initially is that it must be a somewhat easy puzzle, since you obviously don't have to put only one of each number in the diagonals. I say obviously because there are two 1's in the top-right to bottom-left diagonal, and no way to put a 7 on that diagonal.
For the glory of Gondor, I sack this here concession stand!
Or if you just want the highlighted squares: 8, 6, 3 respectively.
Fragment of the Lord of Nightmares, release thy heavenly retribution. Blade of cold, black nothingness: become my power, become my body. Together, let us walk the path of destruction and smash even the souls of the Gods! RAGNA BLADE!
Lore Monkey | the Pichu-master™
Secularism—since AD 80
Av: Elika; Prince of Persia