Why `Zn, Cd` and `Hg` are not considered as typical transitions elements ?

Explanation

Transition elements have incomplete `d`-orbitals. `Zn, Cd` and `Hg` are the end members of the first three series of the transition elements. The electronic configuration of `Zn, Cd` and `Hg` are represented by general formula `(n-1)d^(10) n s^(2)`.
Since in these elements and their ions in stable state, the `d`-orbitals are completely filled hence they do not show general properties of transition elements, e.g.
`Zn = 3d^(10) 4s^(2)` and `Zn^(2+) = 3d^(10) 4s^(0)`
`Cd = 4d^(10), 5s^(2)` and `Cd^(2+) = 4d^(10) 5s^(0)`
`Hg = 5d^(10), 6s^(2)` and `Hg^(2+) = 5d^(10) 6s^(0)`