Shared Styles already existed in Sketch 2, but they have been given a significant overhaul in version 3. Shared Styles are now located in the the white box between the general layer options an the style options.
You can create a new style by selecting a shape and creating a new style from the drop-down menu there. You can create as many styles as you want and you can switch between existing styles using this panel as well.
A change you make to one Shared Style is immediately reflected on all the others.