3D
Space is nothing else than the form of all phenomena of the external sense, that is, the subjective condition of the sensibility, under which alone external intuition is possible. ~ Immanuel Kant
No physics theory demands 3 dimensions as the nominal spatial construct. Why then does existence appear in 3D?
Nature’s love of diversity withstanding, economy is a cardinal facet of existence. All of physics pronounces it: from classical thermodynamics to quantum mechanics. Chemistry cogently coheres about energy parsimony guiding reactivity, despite the plethora of ways in which reactions may transpire.
Time is a single vector. No one questions why there are not 2 dimensions of time, because it is inconceivable how that would work.
Following Occam’s razor, the snappiest answer for 3D space as spot-on is adequacy: 3 dimensions are the fewest that can create a rich life experience.
Extra dimensions may be necessary to explain quantum and cosmological mechanics, but ambient 3D is opulently textured in a way that flatland (2D) could never approach, at the expense of only a single extra dimension.
Conversely, experiencing more than 3 spatial dimensions would create an entirely new level of complexity in experiential existence, and in so doing contravene Nature’s proclivity for economical sufficiency.
