The increasing order of ionization energy among the elements Se,Cl, and S is

Se,Cl, and S
Electronegativity depends on the atomic radii of the element and also on its position in the periodic table.
Cl and S are the members of 2nd period while Se belongs to the 3rd group. As we know electronegativity decreases down the group so, Se would be the least electronegative. On the other hand S belongs to group 16 while Cl belongs to group 17. Electronegativity increases along the period , so Cl would be more electronegative than S.
So the net order is
Cl > S > Se