Na (Sodium), Mg (Magnesium) and Al(Aluminium) are metals. Si (Silicon) is a metalloid. P (Phosphorus), S (Sulphur) and Cl (Chlorine) are non-metals.
As we go from left to right across the third period, metallic character decreases and non-metallic character increases.
The most metallic element in the third period is Na and the most non-metallic element is Cl.