We don't want to put out the full source of the built-in plugins in C3, because in C2 this basically caused ongoing compatibility nightmares, to the extent I'd rather simply block the option of copy-pasting an addon to tweak it, and force us to come up with another way to alter other addons in an extensible way. You can however see the runtime scripts in preview mode, as pointed out. If you have any questions about the SDK I'll try to answer them on the forum.
The point about loading project files is a good question, because it's different in C3. Preview mode doesn't actually host on a server, it keeps everything locally, so there's no direct URL to project files. You can solve this by asking the runtime for the URL to the resource with:
var urlToRequest = this.runtime.getLocalFileUrl(projectFileUrl);[/code:132x49b4]
In preview mode, urlToRequest will be a blob URL that you can fetch normally and will load the project file locally. After export this just passes the URL through and fetches it from the network as usual.