The proton number of an atom does not change when it becomes an ion. Only the electron number changes.
In its neutral state (before becoming charged), an atom should have the same number for protons and electrons.
Therefore, given the element has lost 3 electrons, gaining a positive charge of 3+.
The atomic number of an element is the same as its proton number.
Looking at a periodic table, we can see that the element with the atomic number 13 is aluminum.
To use the proper notation, we would use the atomic symbol of aluminum and place square brackets around it with the charge 3+:
[Al]³⁺
