You can open templates as a new document with pre-populated layers. The Mac app comes with several useful sample templates, including the iOS App Icon and Material Design templates.
You’ll see a grid view of templates in the Workspace window. You can bring this up at any time by choosing Window > Workspace.
You can also get a list of templates by choosing File > New From Template….
To save a document you’ve created as a template, choose File > Save as Template….
Templates are local documents only, and you can find them in
~/Library/Application Support/com.bohemiancoding.sketch3/Templates. This is also where you can save a custom preview image to appear in the Workspace window.
How to Create Custom Template Previews
To give your templates a custom preview, add a PNG image with the same filename as your template to its location in the Finder. For pixel-perfect previews, images should be 188 × 188 px for Retina displays, or 94 × 94 px otherwise.