There were three demons, called Kamalakshan, Tarakakshan and Vindumali. They had been blessed with a boon that they would not be killed unless all three were felled by the same arrow. They built strong forts in different parts of the world and ruled in their respective regions. The celestials complained to Lord Siva about the demons, and the Lord decided to destroy them. The earth itself became His chariot; the sun and moon were the wheels; the mountain Meru was His bow. The Lord however, did not use any weapon. He merely smiled and that was enough to burn down the three places ruled by the demons. Because He burned down three areas, He came to be known as Tripurantaka.
There is a symbolic meaning to this story, explained K. Sambandan, in a discourse. In fact the Saivite saint Thirumoolar said that those who said that Lord Siva burnt three cities were fools. What He actually burns down are illusion, the effects of our karma and our ego. These are the three things that He burns down. His destruction of the three cities, therefore, is to be viewed symbolically as an act of purification. Wherever there is evil, the Lord steps in and destroys it. He is both merciful and fierce. He is fierce when it comes to burning down our undesirable traits. He is merciful when it comes to forgiving us for our wrongdoings. In the story of the three demons, although He destroyed their cities, He forgave them and gave one of them the task of fanning Him. The other two were appointed His guards.