Many of the transition metals' properties comes from the fact that their chemistry involves electrons from d-orbitals. Since Zn, Cd, and Hg have a d10 configuration, most of their chemistry involves only the electrons in their s-orbitals. Therefore, these metals do not exhibit some of the same chemical and physical properties associated with transition metals (e.g. colored ions, multiple oxidation states, ease of oxidation/reduction, etc.).