If accuracy is important, I would use a digital scale to determine the mass of a whole teaspoon. Then, divide that by 3 and use the scale to measure out the ingredient.
How to measure 1/3 teaspoon?
The only time I've heard of using 1/3 teaspoon for measurement is in cooking. Most measuring spoon sets come with a 1/3 teaspoon measure. For anything else, Stormy has the right idea.