The solubility product of CuS,Ag2S and HgS are 10^- 37 ,10^- 44 and 10^-54 respectively the solubility of these sulfides will be in order

For CuS &HgS Ksp=S²..(where s= solubility) & for Ag2S ; Ksp=4S³(s= solubility) now put the values of Ksp and check in which case the value of S is highest & lowest.        
Solubility of CuS : 
(10^-31)^1/2 = ( 10^-31/2); 

Ag2S : 

(10^-44)^1/3 =( 10^-44/3); 

HgS ; 

(10^-54)^1/2 = (10^ -54/2) =(10^-27); 

The order of solubility will be as per above values: 
Hence ,the order of solubility is: 

Ag2S> CuS > HgS