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Deeply subwavelength lasers (or nanolasers) are highly demanded for compact on-chip bioimaging and sensing at the nanoscale. One of the main obstacles for the development of single-particle nanolasers with all three dimensions shorter than the emitting wavelength in the visible range is the high lasing thresholds and the resulting overheating. Here we exploit exciton-polariton condensation and mirror-image Mie modes in a cuboid CsPbBr3 nanoparticle to achieve coherent emission at the visible wavelength of around 0.53 μm from its ultra-small (≈0.007 μm3 or ≈λ3/20) semiconductor nanocavity. The polaritonic nature of the emission from the nanocavity localized in all three dimensions is proven by direct comparison with corresponding one-dimensional and two-dimensional waveguiding systems with similar material parameters. Such a deeply subwavelength nanolaser is enabled not only by the high values for exciton binding energy (≈35 meV), refractive index (>2.5 at low temperature), and luminescence quantum yield of CsPbBr3, but also by the optimization of polaritons condensation on the Mie resonances with quality factors improved by the metallic substrate. Moreover, the key parameters for optimal lasing conditions are intermode free spectral range and phonons spectrum in CsPbBr3, which govern polaritons condensation path. Such chemically synthesized colloidal CsPbBr3 nanolasers can be potentially deposited on arbitrary surfaces, which makes them a versatile tool for integration with various on-chip systems.