What is common between the elements that are found in period 3 of the the periodic table?​

Elements on the left of the period 3 have higher melting and boiling points than those on the right. These low melting and boiling points are typical of non-metals. Argon, chlorine, sulfur and phosphorus are non-metals. On the left hand side of period 3 we find the elements sodium, magnesium and aluminium.

Explanation:

In the whole of period 3, the outer electrons are in 3-level orbitals. These are all the same sort of distances from the nucleus, and are screened by the same electrons in the first and second levels. The major difference is the increasing number of protons in the nucleus as you go from sodium across to argon.

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they have three complete shells

Explanation:

answer is: naturally occurring solid, inorganic compound or element is mineral.

minerals usually have crystalline form, that means than crystal constituents (atoms, molecules or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

for example magnetite is a mineral of iron oxide (fe₂o₃).

calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (caco₃).

dolomite is an   carbonate mineral composed of calcium magnesium carbonate (camg(co₃)₂).