In this symbolical image you can see three Goddesses who are enthroned on the lotuses on the prongs of Shiva's Trident.
Parā (the middle prong of the trishula) is depicted in a benevolent form, while Parāpara (the left prong) and Aparā (the right prong) are shown as wild and terrifying, wearing a garland of skulls, and brandishing the khatvanga, the skull-topped staff. The Three Goddesses are white, red and yellow-black. And we see Sadashiva at the base of the prongs of the trishula.
Parā, beautiful and brilliant white, considered Supreme Shakti. It means the highest form - transcendent, beyond and outside the ordinary range of human experience or understanding. In a broad sense all Great Goddesses, such as Durga, Kali, Tripura can be considered to be Paraa. In order to manifest She loses her supremacy and becomes the mediocre level of supremacy. Parāparā, a medium shakti, is red as blazing fire, wearing a garland of skulls glowing with three eyes. She sits with trident and skull-staff in her hands on Sadashiva, the Great Transcended. She further loses her strength at the exact time of manifestation loses its supremacy and is manifested as Aparā, a lower shakti, the destroyer of the pains of the humble. She looks the same as Parāparā except that She is black. Her mouth yawns wide and has huge fangs. Ferocious, with her brows knitted in rage, wearing a sacred thread in the form of a huge snake, adorned with a string of human corpses round her neck, with the severed lotus hands of a human corpse as beautiful ear-rings, her voice like the thunder of the clouds at the world's end, she seems to swallow space itself. It leads us to a conclusion that the trika of Goddesses are Kali in her immanent form.
Certainly, all of this is full of deep symbolism, used for the convenience of philosophical thinking but we can depict just some general ideas.
All three forms are composed of the 3 Energies of Shiva-Bhairava in the Transcendant, Imminent and Transcendant-Imminent meaning.
They can represent, as their colours suggest, the three qualities-Gunas (Satvic, Rajas, and Tamas) of Nature (prakrti) which the one Goddess embodies.
Traditionally, it can be treated in Trika as three tattvas - Shiva (Absolute Unity), Shakti (link between unity and duality), and Nara (jiva or duality).
Vijnana Bhairava describe this differences as three stages. In the state of Shiva (para) both knowledge and action are blended in equal proportion. In the stage of Shakti only knowledge is predominant and in the stage a-para (human level) only action is predominant. This is known as trika-bhedam.
Also, the three vidyas refer to Parā which rests in the Absolute only as Will to manifest as only Iccha-Shakti; to Parāparā as Jnana-shakti and Kriya-shakti as Aparā.
And in internal personal level, we can see here three vidyas introverted towards inner self: Buddhi, Ahamkara and Manas.
Despite this Trika's triadic-tetradic schema of the three Goddesses, nobody can hide from us their fundamental, indestructible and transcendental unity.
OM NAMAH SHIVAYA