Draw the Lewis structure for the polyatomic hydronium H3O cation. Be sure to include all resonance structures that

 Lewis structure of Hydronium ion is shown below :                          

Explanation:

Lewis structure : It is a representation of valence electrons on the atoms in a molecule

Here , Hydronium ion is given , which contains 1 atom of oxygen and 3 atoms of hydrogen .

Oxygen has a total of 6 valence electrons and hydrogen contains 1 valence electron .

Oxygen share its 3 valence electrons with 3 hydrogen atoms and left with 3 valence electrons. From these three valence  electrons of oxygen atom  two electrons will be shown as a pair of electrons on oxygen atom but a single electron can not be shown . So , to simplify this, one positive charge is shown overall .  

Resonance structure will be same as the hybrid structure because all  three atoms are same , that is hydrogen .