They are also called "the triple gateway to liberation" (vimokkha-mukha; Path of Purification XXI, 66ff) because they are three different approaches to the paths of enlightenment.
1. "Whosoever, being filled with determination, reflects that all formations are radically impermanent, such a person attains the conditionless liberation.
2. Whosoever, being filled with tranquility, reflects that all formations are painful, such a person attains the desireless liberation.
3. Whosoever, being filled with wisdom, reflects that all formations are without a self, such a person attains the emptiness liberation" (Path of Purification XXI, 70 = Pts.M. II, p. 58).
(3) ''By thinking of the beautiful, one is filled with confidence: this is the third liberation.
(6) "Through the total overcoming of the plane of unbounded consciousness, and with the idea 'Nothing is there,' one reaches the plane of nothingness (ákiñeaññ-áyatana) and abides therein: this is the sixth liberation.
According to Pts.M. this mental state is also produced by concentrating the mind on the Four Sublime States (Brahma Viharas): all-embracing loving-kindness, compassion, appreciative joy, and equanimity. The result of this is that all beings appear perfectly pure and glorified, and therefore the mind turns to the beautiful.