OpenLayers 3 or as it is referred OL-3 is a Javascript Library for web mapping, so in order to use it you'll need to add it in your html:
first add the ol.css file to use the map styling of OL-3 :
then add the ol.js file :
you can also download OL-3 from the official site www.openlayers.org and call the files in the html by changing the src and href