I will attempt a cake 3 tiers and 2 layer each. I thought I would transport separately and assemble on site. Now do I need to dowel that cake? Can I just sit them on each other, with cake boards and straws in between of course? but do I need the center dowel?

each tier must have dowels (except the top, of course) You will not need a center dowel if you transport separately.

I have done several 4 tier wedding cakes with no dowels in the tiers. I transport seperately and assemble on site with no problems. If you are unsure, then I would put dowels in the tiers just to be safe.