What binding options are there for an A3 sketchbook that I can rearrange or add pages to?
You can make your own sketchbook ring binder:

That way, new blank A3 pages, as well as coloured index tabs, can easily be added anywhere.
You can perforate new pages yourself using a hole puncher.
If your project files consist of a lot of loose or thick objects, you can also opt for a binder box.