Why `Cu, Ag` and `Au` are transition elementsm although they have completely filled `d`-orbititals ?

Explanation

These elements have completely filled `d`-orbitals but their ions in their stable oxidation state have incomplete `d`-orbitals. Hence they show general properties of transition elements, e.g.
`Cu = 3d^(10) 4s^(1)` and `Cu^(2+) = 3d^(9) 4s^(0)`
`Ag = 4^(10) 5s^(1)` and `Ag^(2+) = 3d^(9) 4s^(0)`
`Au = 5d^(10) 6s^(1)` and `Au^(3+) = 5d^(8) 4s^(0)`