The three isomers are shown in the attached diagram. The structure on the left has no net dipole moment and the structures in the center and on the right do. Here's why:
The structure on the left and the one in the center are isomers of 1,2-dibromoethene. The one on the left is the trans-isomer and the one in the center is the cis-isomer. Both contain polar C-Br bonds, as bromine is more electronegative than carbon. The trans-isomer has the bromines pulling the electron cloud equally in opposite directions so the dipoles cancel and there's no net dipole moment. The cis-isomer has the bromines pulling the electron cloud in directions that are not at 180 degrees from each other so the dipoles don't cancel out.
The structure on the right is 1,1 dibromoethene. Like the cis-isomer, it's asymmetrical and the dipoles caused by the polar carbon-bromine bonds don't cancel out so there's a net dipole moment.