Maria, I've made several *grooms* cakes for informal weddings---
didn't want the stress producing a photo-worthy official "official wedding cake."
Here's a few things I noticed: each layer should be 3" to 4" larger in size. I made a 10" + 8" +6" tiered cake and there just wasn't enough room for fancy decorative touches. It's doable, but not my favorite look. I have a lovely book called "Wedding Cakes" by Mich Turner and hers are typically 6" + 9" + 12". For smaller cakes with ribbon or painted details only, she'll stack 4" + 6" + 8".
You build each cake on its own cakeboard and then set that on a 1/2" board that is covered with fondant matching the cake and ribbon matching either the fondant color (white/ivory/etc) or the wedding color scheme. That way you can either set the tier directly on the stabilizing dowels or on raised pillars if you want to put fresh flowers between layers or on fake fondant colored smaller tiers. That last is my least favorite. Something about the proportion never looks right to me (to have a larger cake tier sitting on top of a smaller one.
The birthday cakes you made for your neighbor twins were nicer than some bakery wedding cakes around here.