The three natures are the imaginary, the dependent, and the perfected.
Whatever is grasped by mental designation is the imaginary nature. Non-entities and the appearances of objects arising in the mind are imaginary. The relationship between name and object, such as grasping the name as the object or mistaking the object as the name, are also imaginary. Outer, inner, fringe and center, big and small, good and bad, space and time, and so on, whatever is grasped by thought is imaginary in nature.
The dependent nature is simply consciousness which arises as subjective and objective poles, based on the habitual tendencies of ignorance. The perfected nature is self-aware, self-luminous, and free from contrivance. The synonyms of the perfected nature are dharmata, dharmadhatu, suchness, and ultimate truth.
The dependent and imaginary natures are equally false and relative. However, it is necessary to separate them into individual categories. The imaginary nature does not exist even on a relative level. The dependent nature exists on a relative level. The perfected nature does not exist on a relative level, yet it truly exists on an ultimate level.
Therefore, imaginary nature exists by designation, and the dependent nature exists as perception. The perfected nature does not exist in either of these two ways, rather it exists in an uncontrived way.
The imaginary nature is non-existent emptiness. The dependent nature is existent emptiness. The perfected nature is ultimate emptiness.
Lord Maitreya said:
If one understands non-existent emptiness, and likewise existent emptiness, as well as ultimate emptiness, it is said that one understands emptiness.
—Jetsun Taranatha, Maha Madhyamaka