We find that three nonstoichiometric cubic binary chromium chalcogenides, namely Cr3S4, Cr3Se4, and Cr3Te4, are stable half-metallic ferromagnets with wide half-metallic gaps on the basis of systematic state-of-the-arts first-principles calculations. We optimize their structures, and then calculate their magnetic moments, electronic structures, formation heats, and elastic moduli and investigate their structural stability and robustness of ferromagnetism against antiferromagnetic fluctuations. Our calculated results show that the three sulvanite phases are structurally stable and ferromagnetically robust, and hence could be realized as epitaxial thin films. We attribute the structural and ferromagnetic stability and the better half-metallicity to their special effective Cr valence 2.667+. These findings will open doors for much more high-performance spintronic materials compatible with current semiconductor technology.