About 1st ionization enthalpy

First ionization enthalpy: Na < Al < Mg

Na (1s2 2s2 2p6 3s1) has the smallest ionization enthalpy. This is because the nuclear charge of Na (11) is smaller than that of Mg (12) and Al (13). Therefore, the outermost 3s electron experiences the weakest nuclear attraction.

Although Al (1s2 2s2 2p6 3s2 3p1) has a greater nuclear charge than Mg (1s2 2s2 2p6 3s2), Al has a lower first ionization enthalpy. The reasons are:
(1) A 3p electron is removed in the first ionization of Al, but a 3s electron is removed in that of Mg. Less energy is needed to removed a 3p electron than a 3s electron, because the 3p electron is in a higher energy level.
(2) The 3s subshell of Mg is completely-filled, and thus exhibits extra stability. More energy is needed to break the completely-filled structure of the 3s subshell.

(Some books and webpages state that Al obtains a completely-filled structure of 3s subshell. In fact, this is incorrect, because the completely-filled structure of 3s subshell of Al exists before and after the first ionization.)
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Na < Mg < Al

This is because their outest electron are in the same shell (third), which means the outest electron experience the same shielding effect. However, the force of attraction from the nuclei are different. the nucleus of Al has a greater charge than the nucleus of Mg (Al has 13 protons and Mg has 12 protons), so the electrons of Al experience a greater force of attraction than the electrons of Mg. This means more energy is required to ionise / remove the outest electron Al than Mg.

The reason apply to Mg and Na, Mg has 12 protons and Na has 11 protons.