Crystallization of colloidal nanocrystals into superlattices represents a practical bottom-up process with which to create ordered metamaterials with emergent functionalities(1-3). With precise control over the size, shape and composition of individual nanocrystals(4-6), various single-and multi-component nanocrystal superlattices have been produced, the lattice structures and chemical compositions of which can be accurately engineered(7-9). Nanocrystal superlattices are typically prepared by carefully controlling the assembly process through solvent evaporation or destabilization(2,10-15) or through DNA-guided crystallization(16-18). Slow solvent evaporation or cooling of nanocrystal solutions (over hours or days) is the key element for successful crystallization processes(10,18). Here we report the rapid growth (seconds) of micrometre-sized, face-centred-cubic, three-dimensional nanocrystal superlattices during colloidal synthesis at high temperatures (more than 230 degrees Celsius). Using in situ small-angle X-ray scattering, we observe continuous growth of individual nanocrystals within the lattices, which results in simultaneous lattice expansion and fine nanocrystal size control due to the superlattice templates. Thermodynamic models demonstrate that balanced attractive and repulsive interparticle interactions dictated by the ligand coverage on nanocrystal surfaces and nanocrystal core size are responsible for the crystallization process. The interparticle interactions can also be controlled to form different superlattice structures, such as hexagonal close-packed lattices. The rational assembly of various nanocrystal systems into novel materials is thus facilitated for both fundamental research and for practical applications in the fields of magnetics(19), electronics(3) and catalysis(20).