The effect of addition of an inert gas like helium at constant pressure $2S{O}_{2_{\left(g\right)}}+O_{2_{\left(g\right)}}\underset{ }{\rightleftarrows }2S{O}_{3_{\left(g\right)}},$ to equilibria will,

Will decrease forward as well as backward reaction 

Have no effect on the equilibrium  

Will shift the equilibrium in the backward direction  

Shift the equilibrium in the forward side