Composition theory, color theory, visual hierarchy.

Can't Unsee. Interactive game training visual alignment and spacing precision.
Handprint Color & Composition. Deepest free color theory resource online. Covers perception and harmony.
Laws of UX. Gestalt principles, visual perception laws (directly applicable to 3D composition).
Neil Blevins Art Lessons. Composition types, tangents, visual hierarchy by a Pixar artist.
See also: Learning, Community & Industry → 3D Inspiration & Showcase
PhotoPills Golden Ratio Guide. Golden ratio, spirals, triangles with overlay diagrams.