Artboards

Artboards in Sketch 3 can also directly be exported, forgoing the need to add extra Slice layers to the document. Just add an export size to Artboard and next time you click Export, Sketch will include the Artboards as well.

When you click on Export for the first time, and you have some Artboards in your canvas, Sketch will assume you want to export those and will make them exportable automatically.