Abstract We explore the multifaceted important features of turbulence (e.g., anisotropy, dispersion, and diffusion) in the three-dimensional (3D) wavenumber domain (k _∥, k _⊥,1, k _⊥,2), by employing the k-filtering technique to high-quality measurements of fields and particles from the Magnetospheric Multiscale Mission (MMS) multi-spacecraft constellation. We compute the 3D power spectral densities (PSDs) of magnetic and electric field fluctuations (marked as PSD(δ B ( k )) and PSD ( δ E 〈 v i 〉 ′ ( k ) ) , respectively), both of which show a prominent spectral anisotropy in the sub-ion range. We give the first 3D image of the bifurcation between the power spectra of the electric and magnetic fluctuations, by calculating the ratio between PSD ( δ E 〈 v i 〉 ′ ( k ) ) and PSD(δ B ( k )), the distribution of which is related to the nonlinear dispersion relation. We also compute the ratio between electric spectra in different reference frames defined by the ion bulk velocity, PSD ( δ E local v i ′ ) / PSD ( δ E 〈 v i 〉 ′ ) , to visualize the turbulent ion diffusion region (T-IDR) in wavenumber space. The T-IDR has an anisotropy and a preferential direction of wavevectors, which is generally consistent with the plasma wave theory prediction based on the dominance of kinetic Alfvén waves. This work demonstrates the worth of the k-filtering technique in diagnosing turbulence comprehensively, especially when the electric field is involved.