The correct structure of tribromooctaoxide is:

Tribromooctaoxide (${\text{Br}}_3{\text{O}}_8$) consists of a chain of three bromine atoms.The two terminal bromine atoms are bonded to 3 oxygen atoms each (oxidation state $+6$).The central bromine atom is bonded to 2 oxygen atoms (oxidation state $+4$).The structure consists of a $\text{Br}-\text{Br}-\text{Br}$ chain with all eight oxygens as double-bonded ligands ($=O$).