Web games have come a long way thanks to WebGL and related HTML5 APIs. The most prominent open source library for 3D is Three.js.
While Three.js is powerful, it’s also complex to learn from scratch. Instead, you can pick up Whitestorm.js, an open source framework made for 3D web graphics. It uses Three.js as an underlying technology to help you build faster and create realistic 3D effects in the browser.