Write the chemical symbols for three different atoms or atomic cations with 18 electrons.

Chlorine has 17 electrons in it natural state but it gains an electron and becomes an anion with 18 electrons. 

Potassium (K) has 19 electrons in its natural state. It loses an electron and becomes K+ and then is left with 18 electrons .

The atomic number of calcium is 20 i.e. Ca has 20 electrons.It Loses 2 electrons and becomes Ca++ and  has 18 electrons.