With 3.0 the cms is now an optional module so yes, you can use it on it's own
If you want to setup routes you need to use Director::addRules() (although this will change in 3.0 stable and move to static YAML definitions) to take all your routes and point them to controllers.
The base controller class (Controller) and RequestHandler handle detecting 404 errors so I would investigate those and pick out the bits you want to customize in your own base custom controller for the application..
I am also interested in using SiteConfig but in the framework only version. As SiteConfig is included in the cms version only could you please advise me if/how I can use this without the cms? I have been extending the framework admin to provide a simple system. Using the cms seems overkill for my project.