As an AI language model, I can provide you with an example of a simple 3D open-world game implemented in a single HTML file using JavaScript. However, please note that creating a fully functional and immersive 3D open-world game requires advanced programming skills and extensive development time. This example will only have basic functionality and serve as a starting point for further enhancements. Here's the code:
In this simple game, a green cube is rendered in a 3D scene, and you can control the camera by tilting your smartphone or using orientation controls. However, please note that the game's functionality is limited, and you would need to extend it significantly to create a full-fledged 3D open-world game.