In a recent advance, Nb₃Cl₈ two-dimensional crystals with a kagome lattice and electronic topological flat bands have been experimentally fabricated (Sun et al. in Nano Lett 22:4596–4602, 2022). In this work motivated by the aforementioned progress, we conduct first-principles calculations to explore the structural, phonon dispersion relations, single-layer exfoliation energies and mechanical features of the Nb₃X₈ (X = Cl, Br, I) nanosheets. Acquired phonon dispersion relations reveal the dynamical stability of the Nb₃X₈ (X = Cl, Br, I) monolayers. To isolate single-layer crystals from bulk counterparts, we predicted exfoliation energies of 0.24, 0.27 and 0.28 J/m², for the Nb₃Cl₈, Nb₃Br₈ and Nb₃I₈ monolayers, respectively, which are noticeably lower than that of the graphene. We found that the Nb₃X₈ monolayers are relatively strong nanosheets with isotropic elasticity and anisotropic tensile strength. It is moreover shown that by increasing the atomic weight of halogen atoms in the Nb₃X₈ nanosheets, mechanical characteristics decline. Presented results provide a useful vision about the key physical properties of novel 2D systems of Nb₃X₈ (X = Cl, Br, I).