Omega-3 fatty acids are essential fatty acids i.e. the human body cannot synthesize them. They are absolutely essential for growth and maintenance of the body function and their deficiency can cause various problems.
They are precursors for both inflammatory and anti-inflammatory molecules (prostaglandins, thromboxanes and leukotrienes) but you should not assume that inflammation is always bad. It is how your immune system works to identify and destroy pathogens.
PCB stands for polychlorinated biphenyls which are aromatic molecules used for industrial applications are known environmental pollutants. Omega-3 fatty acids have no relationship with these molecules. PCB are much more oil-soluble (lipophilic) than water soluble (hydrophilic). When they are dumped in the environment and are taken up in the bodies of different organisms like fishes, they are effectively partitioned into the fatty compartments of the body (because of higher solubility). The fish oils, that are used as one of the primary sources of the omega-3 fatty acid supplements, therefore concentrate the PCBs that the fishes are exposed to, throughout their lives.
NOTE: The terminologies good-cholesterol and bad-cholesterol are just utterly stupid because cholesterol is cholesterol and it has both good and bad effects depending on how much of it is present in the body. It is like saying good water and bad water. LDL or HDL are not really cholesterols; they are proteins that transport lipids. Read more about this here.