Sure, here's a basic implementation of a simple 3D gun game in a single HTML file using JavaScript and enclosed in script tags. This game allows you to control a character and look up, down, diagonally and shoot targets. Please note that this implementation is simplified for the purpose of this example and may not include advanced features.
In this implementation, the game is displayed on a canvas element with a width of 800 pixels and a height of 600 pixels. The player is represented as a red square and can be controlled using arrow keys for movement, 'w' and 's' keys to look up and down, 'a' or 'd' keys to look diagonally, and the spacebar to shoot. The game loop continuously updates the player's position and angle while the `draw` function takes care of rendering the player and other game elements on the canvas.
Please note that this is just a simplified example and additional features, such as enemy targets, ammunition management, scoring, etc., can be added based on your requirements.