`rarr` Ni has electronic configuration `[Ar]4s^(2)3d^(8)`
`Ni^(+2)` has electronic configuration `[Ar]4s^(0)3d^(8)`
It is difficult to remove an electron from `3d^(8).` So, `Ni^(+3)` is difficult to obtain. (Ni has high negative enthalpy of hydration).
`rarr` Cu has electronic configuration `[Ar]4s^(1)3d^(10)`
`Cu^(+)` has electronic configuration `[Ar]3d^(10)`
It is difficult to remove the electrons from `3d^(10)` (stable). So, `Cu^(+3)` is difficult to obtain.
`rarr` Zn has electronic configuration `[Ar]4s^(2)3d^(10)`
`Zn^(+2)` has electronic configuration `[Ar]4s^(0)3d^(10)`
It is difficult to remove the electron from `3d^(10)` (stable). So `Zn^(+3)` is difficult to obtain.
