The combination of non-trivial band topology and symmetry breaking phases gives rise to novel quantum states and phenomena such as topological superconductivity, quantum anomalous Hall effect and axion electrodynamics . Evidence of intertwined charge density wave (CDW) and superconducting order parameters has recently been observed in a novel kagome material AV3Sb5 (A=K, Rb, Cs) that features a Z2 topological invariant in the electronic structure . However, the origin of the CDW and its intricate interplay with topological state has yet to be determined . Here, using hard x-ray scattering, we demonstrate a three-dimensional (3D) CDW with 2 * 2 * 2 superstructure in (Rb, Cs) V3Sb5 . Unexpectedly, we find that the CDW fails to induce acoustic phonon anomalies at the CDW wavevector but yields a novel Raman mode which quickly damps into a broad continuum below the CDW transition temperature . Our observations exclude strong electron-phonon coupling driven CDW in AV3Sb5 and point to an unconventional and electronic-driven mechanism that couples the CDW and the topological band structure.