What is the smallest of three veins capillaries and arteries?

The smallest of the three is capillaries.

Capillaries are the smallest and most numerous of the three types of blood vessels. They have a diameter of only 5-10 micrometers, which is about the same size as a red blood cell. Capillaries are found in all tissues of the body and they are responsible for the exchange of oxygen and carbon dioxide between the blood and the tissues.

Arteries are larger than capillaries and they carry oxygenated blood away from the heart to the rest of the body. Veins are larger than arteries and they carry deoxygenated blood back to the heart.