First, write out algebraically what associativity means. then, draw a Venn diagram to describe each side of the equation separately. See whether each diagram is the same. If they are, it is good reason to believe it is associative, but not a proof. You need to be more rigorous for that

(By the way, Venn diagrams are not used as proof for anything. if you want, you can draw several diagrams for each side, showing different situations of the intersections between the three sets)