In molecule, the formal charges of oxygen atoms 1, 2, 3 are respectively

formal charge, $\mathrm{FC}=\mathrm{V}-\mathrm{N}-\frac{\mathrm{B}}{2}$

where, V = number of valence electrons of the neutral atom in isolation (in its ground state); 

N= number of non-bonding valence electrons on this atom in the molecule, and 

B= total number of electrons shared in bonds with other atoms in the molecule.

The formal charge on the central O atom marked 1= $6-2-\frac{6}{2}=+1$

The formal charge on the end O atom marked 2= $6-4-\frac{4}{2}=0$

The formal charge on the end O atom marked 3=$6-6-\frac{2}{2}=-1$