We report on structural and optical properties of self-assembling GaN and InN three-dimensional (3D) nanostructures grown by a vapor–liquid–solid mechanism on GaAs(311) and Al2O3(0001) substrates. Characterization was done using scanning electron microscopy (SEM), time-resolved photoluminescence (PL) and cathodoluminescence (CL). SEM analysis has shown that the GaN 3D nanostructures grow up in the shape of nano-columns regularly distributed on the planar GaN layer surface, while the 3D InN nanostructures form more complex inverted cones. Luminescence spectra of the GaN 3D nano-columns are dominated by the emissions at 3.46 and 3.26 eV related to donor-bound-exciton and donor–acceptor pair recombination, respectively. CL measurements have revealed two emission lines at 2.4 and 2.2 eV originated from the InN 3D nanostructures.