<p>We explore the multi-faceted important features of turbulence (e.g., anisotropy, dispersion, diffusion) in the three-dimensional (3D) wavenumber domain (k<sub></sub>, k<sub>perp</sub><sub>1</sub>, k<sub>perp</sub><sub>2</sub>), by employing the k-filtering technique to the high-quality measurements of fields and plasmas from multi-spacecraft constellation (i.e., MMS). We compute the 3D power spectral densities (PSDs) of magnetic and electric fluctuations (marked as PSD(&#948;B(k)) and PSD(&#948;E&#8242;&#8249;v<sub>i</sub>&#8250;(k))), both of which show prominent spectral anisotropy in the sub-ion range. We calculate the ratio between PSD(&#948;E&#8242;&#8249;v<sub>i</sub>&#8250;(k)) and PSD(&#948;B(k)), the distribution of which is related with nonlinear dispersion relation. We also compute the ratio between electric spectra in different frames of ion flow, that is PSD(&#948;E&#8242;local v<sub>i</sub>)/PSD(&#948;E&#8242;&#8249;v<sub>i</sub>&#8250;), to demonstrate the turbulence ion diffusion region (T- IDR) in the 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 Alfv&#233;n wave (KAW). This work manifests the worth of the k-filtering technique in diagnosing turbulence comprehensively, especially when the electric field is involved.</p>