First results from the three-fluid 3D model of the solar corona are reported. The model extends previous 2.5D three-fluid studies, and includes electron, protons, and oxygen ions (O5+) as coupled fluids in a tilted dipole solar magnetic field configuration. The model demonstrates the variation in abundances of the oxygen ions relative to protons in a coronal streamer belt, thus identifying the sources of the slow solar wind and helping in the interpretation of past spectroscopic observations.