We know that as according to concept of Hardy's schulze law states that the amount of electrolyte required for the coagulation of a definite amount of the solution is dependent on the valency of the coagulating ion.  Coagulation is generally carried out by the addition of electrolytes.

Therefore, higher is the valency of the flocculating ion or coagulating ion, greater is its power to coagulate.

Hence, the correct order should be 

${\mathrm{MgCl}}_2={\mathrm{BaCl}}_2>\mathrm{NaCl}$