Explain the formation of Al3+ ion and why is it formed?

 Atomic number of the Al =13

Electronic configuration of Al = 2, 8, 3
Al³⁺ is formed by removal of 3 electrons from Al atom.

 Electronic configuration of Al³⁺= 2, 8
As we know the most stable elements in periodic table are noble gas. They have 8 electrons in their valence shell which is known as octet. 
To acquire the noble gas configuration Al loose its 3 electrons to form Al³⁺.