What are the oxidation states of three Br atoms in Br 3 O 8 molecule

Oxidation number or state of an atom/ion is the number of electrons an atom/ion that the molecule has either gained or lost compared to the neutral atom. In molecules, more electronegative atom gain electrons from a less electronegative atom and have negative oxidation states. The numerical value of the oxidation state is equal to the number of electrons lost or gained.

The termianal bromine atoms are attached with three oxygen each. Each oxygen oxidation state is $-2$. Hence terminal bromine xoidation state is $+6$. The bromine in the middle is attached with two oxygen atoms. Hence, its oxidation state is $+4$.