As far as I understand, Stable Cascade is a pretty large architecture change from the existing Stable Diffusion models. Sable Diffusion 3 will likely be an interitive improvement with similar architecture over the existing Stage Diffusion models. The blog post for Stable Cascade does a better job explaining the difference than the Stable Diffusion 3 blog post does. Here's a god quote from that post:

Stable Cascade differs from our Stable Diffusion lineup of models as it is built on a pipeline comprising three distinct models: Stages A, B, and C. This architecture allows for a hierarchical compression of images, achieving remarkable outputs while utilizing a highly compressed latent space.

Hope this helps!