Chlorine, bromine and a hydroxyl group are attached to three different carbons of a benzene ring (compounds $\mathrm{C}{}_6\mathrm{H}{}_3\mathrm{Br}(\mathrm{Cl})\mathrm{OH}$). The number of structural isomers is

Three different substituents: the 1,2,3 pattern gives 3 (which one is in the middle), the 1,2,4 pattern gives 6, the 1,3,5 pattern gives 1. Total 3 + 6 + 1 = 10, confirmed by placing the three groups on all ring positions and removing equivalent structures.