The time that each solid object takes to cool depends on its surface area. The solid object with the smallest surface area would cool fastest, as heat is lost in proportion to the surface area. When comparing the surface areas of the shapes mentioned in the question, the sphere always has the smallest surface area in comparison to a cube or a disc of the same radius. This is due to the geometrical property of the sphere, where surface area-to-volume ratio is minimum.

Option 1: Circular Plate - A circular plate has a larger surface area than a sphere of the same radius. It will take more time to cool.

Option 2: Cube - A cube also has a larger surface area compared to a sphere of the same edge length. It will take more time to cool.

Option 3: Sphere - As the sphere has the minimum surface area amongst all the shapes given, it will cool the fastest.

Option 4: This option is incorrect because heat dissipation rate depends on the surface area and not all of them have the same surface area.

Hence, the correct answer is option 3.