Solution

The group having triangular planar structures is $$\displaystyle CO^{2-}_3 NO^-_3, SO_3$$
In $$\displaystyle CO^{2-}_3 $$ ion, C atom is $$\displaystyle sp^2$$ hybridised. This results in triangular planar structure.
In $$\displaystyle NO^-_3$$ ion, N atom is $$\displaystyle sp^2$$ hybridised. This results in triangular planar structure.
In $$\displaystyle SO_3$$, S atom is $$\displaystyle sp^2$$ hybridised. This results in triangular planar structure.
In all above molecules/ions, the central atom has 3 bonding domains and bond angle of $$\displaystyle 120^o$$.

Note:
$$\displaystyle NCl_3 \: NF_3 \: and \: NH_3$$ are pyramidal.
$$\displaystyle BCl_3 \: and \: BF_3$$ have triangular planar structure.