Among the following, find out the total number of black metal sulphides.

$\mathrm{CuS}; \mathrm{NiS}; \mathrm{CoS}; \mathrm{ZnS}; \mathrm{CdS}$

The black metal sulphides are,

$\mathrm{CuS}; \mathrm{NiS}; \mathrm{CoS}$

Copper sulphide, Nickel sulphide and Cobalt sulphide are known as the black metal sulphides.  

Zinc sulphide is insoluble in water and can be made by treating zinc(ll) salt with hydrogen sulphide.
This is the reason why zinc sulphide does not have black color.
Cadmium sulphide is a yellow solid.

So, the correct answer is 3.