Option 1 states that convex mirrors are used by doctors to examine the oral cavity. This statement is incorrect. Dentists typically use concave mirrors to examine the oral cavity as they provide a magnified and clear image of the teeth and gums.

Option 2 suggests that concave mirrors are used as reflectors. This statement is generally correct. Concave mirrors are known for their reflecting properties and are commonly used in various applications such as headlights, telescopes, and solar cookers.

Option 3 states that convex mirrors are used as reflectors. This statement is correct. Convex mirrors are widely used as reflectors, primarily in situations that require a wider field of view, such as in rear-view mirrors in vehicles and some security mirrors in stores.

Option 4 claims that convex mirrors should be used for shaving. This statement is incorrect. Concave mirrors are usually used for shaving as they provide a magnified and clearer reflection of the face.

In summary, option 3 is the correct statement as convex mirrors are indeed used as reflectors.