Group 16 elements contain 6 electrons in their valance shell while group 17 elements contain 7 and group 18 contain 8.
the electronic configuration of the elements are :
sulphur : 2,8,6
chlorine : 2,8,7  and 
argon : 2,8,8

sulphur , chlorine and argon contain 6,7,8 valence electrons respectively.
on this basis the elements are positioned in different groups.

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The positions of elements on the periodic table is determined by their electronic configurations.
especially elements are placed in particular groups based on their valence electrons.
(valence electrons : the electrons in the outermost shell of an atom)
so the electronic configuration of the mentioned elements :
sulphur : (2,8,6)
the no of valence electrons is 6, so it is placed in the 16th group.
chlorine : (2,8,7)
the no of valence electron is 7,so it is placed in 17th group.
Argon : (2,8,8)
the no of valence electron is 8, so it is placed in 18th group.