Formal charge = (valence electrons) $-$ (lone pair electrons) $-\frac{1}{2}$(bonding electrons).

For atom (2), doubly bonded to central O with two lone pairs: $6-4-\frac{1}{2}(4)=0$.

For atom (1), the central O with one lone pair and a single + a double bond: $6-2-\frac{1}{2}(6)=+1$.

For atom (3), singly bonded to central O with three lone pairs: $6-6-\frac{1}{2}(2)=-1$.

So the formal charges on atoms 2, 1 and 3 are 0, +1 and $-1$ respectively.