Answer: There are three positively charged protons in the nucleus.
Step-by-step explanation:
Atomic number is defined as the number of protons or number of electrons that are present in an atom in an electrically neutral atom.
Atomic number = Number of electrons = Number of protons
Ions are formed when an atom looses or gain electrons.
When an atom looses its electron, it results in the formation of a positive ion which is known as cation and when an atom gains electron, it results in the formation of a negative ion known as anion.
Thus if an atom has a charge of -3 , it means the atom has gained three electrons and thus for the atom to be electrically neutral, it must contain three protons which are positively charged and present in the nucleus.