The correct order of Sulphur oxygen bond order in SO 3 ,   S 2 O 3 2 - and SO 4 2 - are

The ${\mathrm{SO}}_3$ has the bond order of two.

The bond order of ${{\mathrm{SO}}_4}^{2-}$:-

It has the four resonating structure. It has the four equivalent oxygen atoms and six bonds between the sulphur and oxygen.

Bond order = $\frac{6}{4}=1.5$

The bond order of ${\mathrm{S}}_2{\mathrm{O}}_3^{2-}$:-

It has the three resonating structure and there are four bonds formed between the sulphur and oxygen atoms. 

Bond order = $\frac{4}{3}=1.33$

Hence, the correct order of Sulphur oxygen bond order is ${\mathrm{S}}_2{\mathrm{O}}_3^{2-}<{\mathrm{SO}}_4^{2-}<{\mathrm{SO}}_3$.