There are many more that three visible wavelength in the visible EM spectrum, and yet we can model any color using only three primary RGB wavelength. Perception of an arbitrary color is equivalent of perception of a linear combination of only three fixed wavelengths that correspond to Red, Green, and Blue.

On the other hand, as far as I know, there seem to be no auditory "primary colors": you cannot fool an ear into believing that it hears an arbitrary frequency by starting with a small finite number of primary frequencies.

The question is about human olfactory sense: is it possible to fool the nose into believing it smells an arbitrary odor by offering a combination from a small collection of "primary smells"?