AI've never worked with cake dummies before, I'm making a wedding cake it's three tier. The top tier is going to be real cake on top of the bottom two tiers that are cake dummies. My questions are do I dowel them? And also should I place the top tier on when I get to the wedding? I'm afraid the car ride will make the top tier sink into the cake dummies or will it be OK for delivery?

No, you don't need to dowel them. Just glue them together using royal icing, and glue your cake on with royal icing too underneath the board it's on. It would probably be ok for delivery, it would depend on the sizes of the dummies and cake, just to check it's not top heavy. But it should be fine. 

You could stand on a dummy, and it won't break, don't worry!

I put wooden skewers through them to keep them together, and you could drive them skewers into the cake drum to attach the dummies to the drum too. That way they won't shift around. Or hot glue them together. @Dayti is right, you don't have to use dowels at all, just attach the board that the real cake is on to the dummy and it will be like a cake stand.