Do I need to do a wooden dowel when stacking 3 tiers? For 2 tiers ( 6" & 8") I always do bubble tea straws. It seems to work fine if I have the straws cut properly. Soon, I'm adding a 10" bottom tier. I plan on supporting with bubble tea straws again. Should I do a dowel also?

Yes, if you're talking about a center dowel through all three tiers, and if transporting.
http://www.wilton.com/cakes/tiered-cakes/stacked-tiered-cake-construction.cfm

Definitely a dowel through the middle, all the way to the bottom. I get nervous transporting anything over 2 tiers without some sort of plate system. SPS is awesome.