For questions like this, first you need to identify the structure of the monomer used in making the polymer. In this case PVC is polyinyl chloride and the monomer is vinyl chloride which has the chemical formula CH2=CHCl where one of the hydrogen from ethylene molecule is substituted with Chlorine. Also know that PVC is an addition polymer which means the double bonds are reactive site for polymer to grow. So to show 3 repeating units, all you have to do is -(CH2-CHCl-CH2-CHCl-CH2-CHCl)-
I hope this helps.