A gonioscope with a 3-mirror design is a specialized ophthalmic instrument used for examining the anterior chamber angle of the eye. The anterior chamber angle is where the cornea and iris meet, and it's important for drainage of aqueous humor, the fluid that nourishes the front of the eye. The three mirrors in the gonioscope allow the examiner to visualize different portions of the anterior chamber angle without having to rotate the device or reposition it. Each mirror provides a different angle of view, allowing for a comprehensive assessment of the entire angle. This is particularly useful in diagnosing and managing conditions like glaucoma, where the drainage angle plays a crucial role.