To determine the electronic configuration of ions, electrons are first removed from the outermost shell (4s) and then from the penultimate shell (3d) .

1. Cobalt (Co): Atomic number $Z=27$. Ground state configuration: $[Ar] 3d^7 4s^2$. For $Co^{3+}$, remove 3 electrons (2 from 4s and 1 from 3d): $[Ar] 3d^6$.

2. Nickel (Ni): $Z=28$. Ground state: $[Ar] 3d^8 4s^2$. For $Ni^{3+}$: $[Ar] 3d^7$.

3. Manganese (Mn): $Z=25$. Ground state: $[Ar] 3d^5 4s^2$. For $Mn^{3+}$: $[Ar] 3d^4$.

4. Iron (Fe): $Z=26$. Ground state: $[Ar] 3d^6 4s^2$. For $Fe^{3+}$: $[Ar] 3d^5$.

Thus, $Co^{3+}$ corresponds to the $[Ar] 3d^6$ configuration.