In Sketch 3, when dragging out a ruler, is there a way to easily calculate thirds? This means you'll have two rulers.
If you create a rectangle and type in 33% for its width then center it on your artboard and select it you can easily add 2 rulers that snap to each side of the rectangle. That will leave you with an artboard divided into thirds.
You can also use the layout settings:
View -> Layout Settings. Then set the number of columns to 3 and gutter width to 0.