Rapid halide anion exchange easily occurring in metal-halide perovskite nanoparticles (NPs) makes it nearly impossible to create a single three-dimensional (3D) CsPbX3 (X = Cl, Br, I) NP that simultaneously comprises two separate perovskite components. To circumvent this problem, we first propose a Ni2+-mediated halide anion-exchange strategy in zero-dimensional (0D) Ni2+-doped Cs4PbBr6 (Cs4PbBr6:Ni) perovskites to achieve ultra-stable 3D CsPbX3 NPs with two coexisting different perovskite individuals of CsPbCl3 and/or CsPbBr3. By combining the experimental results with first-principles calculations, we confirm that the completely isolated [PbBr6]4− octahedra in 0D Cs4PbBr6:Ni NPs can restrict rapid halide anion exchange and the anion diffusion preferentially proceeds in the proximity of substitutional NiPb centers, namely [NiBr6]4− octahedra in a meta-stable state, rather than in the 0D Cs4PbBr6 and residual 3D CsPbBr3 regions, thereby delivering intrinsic dual-band excitonic luminescence from a single 3D CsPbX3 NP with a more stable and efficient CsPbCl3 component as compared to its counterparts synthesized using conventional methods. These new insights regarding the precise control of halide anion exchange enable the preparation of a new type of high-efficiency perovskite materials with suppressed anion interdiffusion for prospective optoelectronic devices.