What does the 3 indicate in Al 3 plus?

Answer 1

Thagt it has lost three electrons.

Aluminium metal has 13 protons(+) and 13 electrons(-), as the neutrally charged atom.

However, as the aluminium cation it has 13 protons(+) and 10 electrons(-) in the charged ion.

(+)13 (-) 10 = (+)3

Hence its ionic charge is Al^(3+).

Answer 2

The 3+ indicates that Aluminum has a positive charge of three due to the fact that it has lost three ions.