The first three spheres of Kether, Chokmah and Binah make the Supernal Triangle and are the basic substance of the whole Tree of Life, the starting points of the three pillars of manifestation: the Pillar of Mildness, the Pillar of Mercy and the Pillar of Severity. Kether is the neutral point initiating the pillar of moderation (Mildness), Chokmah is the beginning of the active, all-encompassing, positive force of the right pillar of Mercy, and Binah is the beginning of the passive, restrictive, negative force of the left pillar of Severity.
Pillar of Severity
Pillar of Mildness
Pillar of Mercy
They work purely at the causative level of events, live in the abstraction of ideas only, while the following seven spheres (Chesed, Geburah, Tiphareth, Netzach, Hod, Yesod and Malkuth) are the basic archetypes of action and result.
The place of the Supernal Triangle is called the The Macroprosopus, the realm of the potential as opposed to the Microprosopus as the realm of the actual. Note that the Microprosopus only consists of Chesed, Geburah, Tiphareth, Netzach, Hod and Yesod. Malkuth, the bottom sphere, has a special status being called 'the bride of Microprosopus'.
Between the Macroprosopus and the Microprosopus we have the space that Qabalists call the Abyss. This is the demarcation between the creators and the created, the mere ideas and their actual forms, the pure abstract and the realized matter. Actual manifestation as we can conceive it begins with Microprosopus, with its first aspect of being in Chesed.
In the Abyss we can find Daath, the invisible Sephirah that translates as 'Knowledge'.
The Ethical Triangle is composed of the three spheres of Chesed, Geburah and Tiphareth. It is the second triad in the Tree of Life, the reflection of the Supernal Triangle on a lower arc, closer to our perception and comprehension, it forms the evolving individuality and its spiritual soul. The balance between Chesed and Geburah finds its equilibrium in Tiphareth.
The Astral Triangle consists of the Sephiroth Netzach, Hod and Yesod, with Netzach being the instinctive emotion ruled by Venus, Hod the intellectual mind ruled by Mercury and Yesod the sphere of the Moon where both aspects are (hopefully) balanced and combined. Yesod also is the bridge between Malkuth as our very Earth and the upper part of the Tree of Life, it is through Yesod that we can get in touch with the spiritual.
While the Ethical Triangle forms the individuality the Astral Triangle forms the personality of the Tree.
As mentioned above, Malkuth is not considered part of the Microprosopus but is called the 'Bride' of Microprosopus. This has a nice relation to the old fairy-tale in which the Prince (the Microprosopus in general and Tiphareth in particular), who is the son of The Great Father (Chokmah) and The Great Mother (Binah) takes a bride into his house to start the circle of procreation anew. (We forget for a while that in this case it is the Prince that moves in.. ;)
Malkuth as the sphere of the Earth is the place where all the emanations from the Tree of Life manifest and become real, as opposed to the purely spiritual and/or astral levels above.