${\mathrm{CO}}_3^{2-},{\mathrm{NO}}_3^{-},{\mathrm{BO}}_3^{3-}$ The correct increasing order of extent of pi-bonding in above molecules are I II III 

In ${\mathrm{CO}}_3^{-2}$ Bond order = 1.33
In ${\mathrm{NO}}_3^{-}$ Bond order = 1.33
In ${\mathrm{BO}}_3^{-3}$ Bond order = 1.00 
 

small sized atoms have better pi bonding capacity than large sized atom, atomic size order C>N>O