i have to make 3 tier cascade  cake for 15-20 persons.i am confuse should i use dowel for this cake ..image is below ..but small and three tiers only.....please can any one guide me..thank you

Well, an 8,6,4 is 40 party servings, and that's a pretty dinky cake.  Maybe you want to do a dummy tier, or 2, [80% the cost of a real ones] .

Yes, you ALWAYS have to use supports--cake board under each cake and then large bore straws-- when you stack real cakes.