Why Copper and Nickel dont have +3 oxidation state?

i didn't get you. kindly elaborate.

Cu → [Ar] 3d¹⁰ 4s¹
Now acc.to the electronic configuration, valency is 1.
But the 4s-orbital can take an electron from 3d, resulting in [Ar] 3d⁹ 4s² , helping it gain a +2 oxidation state...
Now you cannot accommodate more that 2 electrons in that 4s orbital, and so +3 oxidation state is extremely unstable, hence doesnt exist..

Ni -> [Ar] 4s¹ 3d⁹