<p>A three-dimensional mold model was developed to investigate the multiphase coupling phenomena involving fluid flow, heat transfer, and solidification in the meniscus region. This model captures the complete process of slag behavior, from initial entry and spreading to full coverage of the mold, providing a three-dimensional perspective on slag dynamics. It was further employed to reveal the characteristics of slag film formation and distribution, as well as to analyze the complex infiltration behavior of the flux and its correlation with dynamic variations in slag film thickness during mold oscillation. The results show that liquid slag firstly flows into corner regions to form initial slag films, then spreads laterally toward wide/narrow faces while extending longitudinally. However, slower longitudinal solidification delays a complete slag film system compared to the transverse direction. Under mold oscillation, a little liquid slag bypasses corners into wide/narrow faces, forming valley-shaped channels. The wide-face slag rim is thicker, the thickness of slag rim gradually begins to decrease at the onset of negative stage, then increase when it is over. In 3D model at stage 1, liquid slag near the meniscus flows to the inlet, while flow near the slag rim shifts with mold movement, it is a balancing process. During negative strip, both regions direct slag to the inlet. In the stage 3, slag flow from the slag rim moves away, while meniscus-side flow persists but decreases. Compared with the 2D model, the 3D model also shows that the transient inflow rate first dips then peaks when the mold oscillates to the lowest position.</p>