In Sketch 3, you can reset the origin point on rulers by double clicking on the box in the upper left where the rulers intersect.
The process is explained on the Bohemian Coding site here along with other ruler/grid tips:
Sketch has Rulers, which are hidden by default. As we’ve said before, Sketch has an infinite canvas and because of this the Rulers are not fixed; you can grab and drag the ruler around to define your own zero origin:
If you need to reset the ruler origin, just double-click the ruler intersection area:
You can click anywhere on the ruler to add manual guides, and they’ll stay visible as long as the Ruler is visible. You can move all the guides at once by moving the ruler. To move a single guide, you have to grab it inside the ruler, and then you can drag it around. To remove a guide, drag it out of the view and you’ll see it’ll disappear with a poof: