Dense monolithic (Ti,Zr,Hf)C/SiC ceramic nanocomposites with four different molar ratios of metallic elements in the (Ti,Zr,Hf)C phase (i.e., Ti : Zr : Hf = 1 : 1 : 1, 2 : 3 : 5, 2 : 3 : 3, and 1 : 2 : 1) were prepared upon pyrolysis of novel (Ti,Zr,Hf)-containing single-source precursors (SSPs), followed by spark plasma sintering (SPS). A thorough characterization was conducted to elucidate the synthesis of the SSPs, polymer-to-ceramic transformation, chemical/phase compositions, and microstructure of the SiTiZrHfC-based ceramics. The results revealed the feasibility of synthesizing nanocomposites with high (Ti,Zr,Hf)C contents using the SSP method. These nanocomposites were characterized by a unique microstructure with in situ generated (Ti,Zr,Hf)C@C core–shell nanoparticles homogeneously mixed with β-SiC. The ablation behavior of the nanocomposites was evaluated on an air-plasma device for 60 s. Impressively, the nanocomposites exhibited excellent ablation resistance, and the lowest linear ablation rate reached −0.58 μm/s at 2200 °C. Notably, the ablation resistance can be dramatically improved by precisely tailoring the atomic ratios of metal elements within the (Ti,Zr,Hf)C phase via the molecular design of the SSPs. The formation of a multiple-oxide layer with both a high-melting-point phase ((Ti,Zr,Hf)O₂) and low-melting-point phases ((Zr,Hf)TiO₄) and glassy SiO_2, as well as their structure, played a critical role in the enhanced ablation resistance. The uniform distribution of the high-melting-point (Ti,Zr,Hf)O₂ nano/microparticles throughout the glassy SiO₂ matrix significantly enhanced the viscosity and stability of the oxide layer by the pinning effect, offering superior protection against the ingress of oxygen atoms and excellent resistance to mechanical erosion.