Solubility of NaCI, Na₂SO₄ and Na₃PO₄ in water in increasing order is

Larger the ionic character, larger the solubility in water. Smaller the cation, larger the anion, larger the charge larger the covalent nature and thus, smaller the solubility.
Cation is identical 
Charge polarizing power $\stackrel{{\mathrm{Cl}}^{-}<{\mathrm{SO}}_4^{2-}<{\mathrm{PO}}_4^{3-}}{\to }$
Covalent nature $\stackrel{\mathrm{NaCl}<{\mathrm{Na}}_2{\mathrm{SO}}_4<{\mathrm{Na}}_3{\mathrm{PO}}_4}{\to }$
Solubility $\stackrel{{\mathrm{Na}}_3{\mathrm{PO}}_4<{\mathrm{Na}}_2{\mathrm{SO}}_4<\mathrm{Na}Cl}{\to }$