Zn, Cd, Hg have lowest melting points in their respective periods. This can be explained based on
 

When one or more unpaired electrons are present, metallic bond is formed. Increase in unpaired electron makes the metallic bond stronger.

 Electronic Configuration of zinc(30) = [AR]3d¹⁰4s²

Electronic configuration of Cadmium(48)= [Kr]4d¹⁰5s²

Electronic configuration of Mercury= [Xe]4f¹⁴5d¹⁰6s²

Now strong metallic bond is not possible because unpaired electrons are absent.