Using SketchUp Models

Sketchup → Collada

1. Click on the menu item File > Export > 3D Model....
2. Choose COLLADA File (.dae) as export type.
3. If you want to export the textures, click on Options and make sure that Export Texture Maps is checked.
4. Click on Export.

SketchUp will create a Collada file named your-model.dae and a folder named your-model that contains the model's textures.

Collada → Three.js

Make sure you download ColladaLoader.js (find it somewhere by browsing the source) and load it into your HTML project:

<script type="text/javascript" src="Three.js"></script>
<script type="text/javascript" src="ColladaLoader.js"></script>

Use at least r47 since the Collada loader in r46 has a known bug.

var loader = new THREE.ColladaLoader();
loader.load('path/to/your-model.dae', function (result) {
  scene.add(result.scene);
});

Note that this won't work locally, because it loads the Collada file and the textures using AJAX. Therefore you have to serve the files with HTTP or change your browser's security settings as explained in How to run things locally.