Heavy water is deuterium oxide. It is not H3O. H3O can exist only as an ion (the hydronium ion) .

Heavy water is H2O, just like plain old water. It is `heavy' because each of the hydrogen atoms has an additional neutron. The additional neutrons do not affect the chemical properties of heavy water, but they do give it a greater molecular mass, and cause it to block stray neutrons better (hence its use as a moderator).