Question

Periodic Table

Answer 1

I'm going to guess it is because of the energy difference between the s and p orbitals. Obviously, its much easier to remove an electron from each of them, forming a stable noble gas configuration.
the 3+ means that you have removed 3 electrons from the atom.
As you know, most elements want to have a noble gas electron configuration (it has a full valence shell)

Although the last part of the question should be asking why can you have more or less instead no more or less because you CAN have Al\(^{2+}\) and Al\(^{1+}\), although they are not as abundant as the 3+ stable cation.