I've never really worked with dowels and I'm making a 3D tank cake. The bottom is 3 thin layers and the upper tank part (that holds the canon) is a double layer. The top layer is smaller than the bottom, of course.. just picture a tank. I'm wondering if I should use dowels? If so, how many? If not, will it sink? Do I also need a small base in between as well or instead? Cardboard? If not, then what? I would appreciate any advice in this area? Thanks!

There is a possiblity that it could sink. Make sure you dowel every 4 inches. I like to use bubble tea straws and cardboard boxes.

I would say yes. You need dowels. Cut some cardboard to the size you'll need, cover it with foil saran or shelf coving/contact paper.

Then make the caken fill crumb coatn place wooden dowels in until 1/4" is sticking up so that your fingers aren't coated with icing when you put the upper cake on (the weight of the upper cake will push the dowels the rest of the way in)

If you are transpoeting I also suggest a center dowel.
Hth!