The given problem deals with the use of a convex mirror on a moving automobile, with a truck following at a constant distance. We are required to calculate the position, size, and nature of the image formed. Let's break down the solution into different sections for clarity.

Understanding the situation:
- A convex mirror is a diverging mirror that bulges outward.
- The radius of curvature of a convex mirror is always positive.
- The image formed by a convex mirror is always virtual, diminished in size, and upright.
- The position and size of the image depend on the position of the object, the radius of curvature of the mirror, and the distance between the object and the mirror.

Given data:
- Radius of curvature (R) = 2m
- Distance between the truck and the mirror (d) = 3m

Determining the position of the image:
- Since the truck is behind the mirror, the object is virtual.
- The image formed by a convex mirror is located between the focal point and the center of curvature.
- For a convex mirror, the focal length (f) is half the radius of curvature (f = R/2).
- Therefore, the focal length (f) is 1m.
- Since the object is virtual, the image will also be virtual and located behind the mirror.
- The image will be formed at a distance equal to the focal length (1m) from the mirror's surface.

Determining the size of the image:
- The size of the image formed by a convex mirror is always diminished compared to the object.
- The exact size of the image can be determined using the magnification formula: magnification (m) = -di/do, where di is the image distance and do is the object distance.
- In this case, the object distance (do) is the distance between the truck and the mirror, which is 3m.
- The image distance (di) is the distance from the mirror to the image, which is equal to the focal length (1m).
- Therefore, the magnification (m) = -1/3.
- Since the magnification is negative, the image is inverted.

Summary:
- Position of the image: The image is formed at a distance of 1m behind the mirror's surface.
- Size of the image: The image is diminished in size compared to the object, with a magnification of -1/3.
- Nature of the image: The image is virtual, diminished, and inverted.

Overall, the image formed by the convex mirror on the moving automobile is located 1m behind the mirror's surface, diminished in size with a magnification of -1/3, and inverted.