Pólya's Enumeration formula and isomers 
The hydrocarbon benzene has six carbon atoms arranged at the vertices
  of a regular hexagon, and six hydrogen atoms, with one bonded to each
  carbon atom. 

I know that two molecules are said to be isomers if they are composed
of the same number and types of atoms, but have different structure.
How many isomers may be obtained by replacing two of the hydrogen
atoms with chlorine atoms, and two others with bromine atoms? 
Show that exactly three isomers (ortho-dichlorobenzene, meta-di-chlorobenzene, and para-dichlorobenzene) may be constructed by replacing two of the hydrogen atoms of benzene with chlorine atoms.
So far I've ended up nowhere, I would appreciate the help. 

For the dichlorbenzene, observe that the two chlorine atoms may be adjacent or diagonally opposite or at distance 2.
The possible distributions of chlorine and bromine are (up to rotation and reflection)


*

*Cl Cl Br Br H H

*Cl Cl Br H Br H

*Cl Cl Br H H Br

*Cl Cl H Br Br H

*Cl Br Cl Br H H 

*Cl Br Cl H Br H

*Cl H Cl Br Br H

*Cl H Cl Br H Br

*Cl Br Br Cl H H

*Cl Br H Cl Br H

*Cl Br H Cl H Br

