When elements reaction to form compounds they gain, lose or share valence (outer) electrons so has to achieve a full valence level which results in stability. Argon, being a noble gas, is very inert and has no stable compounds. It exists as an atom in the atmosphere. This is because it already has eight valence electrons, which is a complete octet (full outer level.) It's the least reactive of the three. Chorine is the most reactive of the three because it has seven valence electrons and only needs one more for a complete octet. It gains one electron very readily. It exists as a diatomic gas in its elemental form, sharing a pair of electrons between the two atoms. Sulfur is more reactive than argon but less reactive than chlorine. It has six valance electrons and needs to gain two for a complete octet. An element with 7 valance electrons is more reactive than one with 6 valance electrons because the 2nd electron affinity for an atom is lower than the first. This means that less energy is released when a second electron is gained compared gaining the first. Sulfur exists as a free element in nature as well as in compounds.