In this study, biochar (BC), triple superphosphate (TSP), and TSP+BC amendments were utilized for remediation of Cd, Pb, and Cu co-contaminated agricultural soils. The toxicity characteristic leaching procedure (TCLP), the European Community Bureau of Reference (BCR), X-ray diffraction, and scanning electron microscopy-energy dispersive spectrometer techniques were employed to evaluate the effectiveness of the three types of amendments. After soil amendment, pH, heavy metal concentrations in TCLP extracts, and BCR speciation of heavy metals showed significant changes. The application of BC, TSP, and TSP+BC to co-contaminated soils slightly increased soil pH; decreased Pb, and Cu leachability in the TCLP extracts; and lowered the concentrations of the acid-soluble fraction of heavy metals. The application of TSP+BC mixture at the same dose as BC and TSP produced the greatest reduction in available heavy metal concentration. The optimum mass ratio of TSP to BC was 1:3. Overall, the TSP+BC mixture was highly effective in immobilizing Cd, Pb, and Cu in co-contaminated agricultural soils. The experimental results demonstrate that the rational application of the TSP and BC provides benefits of retrenching phosphorus resources, decreasing phosphorus pollution, and lowering the feed costs of debasing soil remediation treatments.