The number of isoelectronic species among Sc^ 3+ , Cr^ 2+ , Mn^ 3+...

For the complex CoCl₃(en)₂NH₃, cobalt is in +3 oxidation state with electron configuration [Ar]3d⁶.
In an octahedral field with strong field ligands (ethylenediamine, ammonia, chloride), Co³⁺ forms a low-spin complex with $t_{2g}^6 e_g^0$ configuration.
The $t_{2g}$ orbitals consist of $d_{xy}$, $d_{xz}$, and $d_{yz}$.
With all 6 electrons filling the three $t_{2g}$ orbitals, the total number of electrons in the $t_{2g}$ orbital set is 6.