The most stable ion of aluminum carries a charge of +3. This happens because aluminum is in group 13 of the periodic table and has three electrons in its outer shell. When it forms an ion, aluminum tends to lose these three electrons to achieve a stable electronic configuration, resembling that of the nearest noble gas, neon.
When aluminum loses these three electrons, it becomes positively charged, giving it the +3 charge. This +3 charge is what allows aluminum to bond effectively with nonmetals, such as oxygen, to form compounds like aluminum oxide (Al₂O₃). Therefore, the most stable state for aluminum in its ionic form is indeed as Al3+.