Doping and electrical characteristics of in-situ heavily B-doped Si1-x-yGexCy (0.22<x<0.6, 0<y<0.02) films epitaxially grown on Si(100) were investigated. The epitaxial growth was carried out at 550 °C in a SiH4-GeH4-CH3SiH3-B2H6-H2 gas mixture using an ultraclean hot-wall low-pressure chemical vapor deposition (LPCVD) system. It was found that the deposition rate increased with increasing GeH4 partial pressure, and only at high GeH4 partial pressure did it decrease with increasing B2H6 as well as CH3SiH3 partial pressures. With the B2H6 addition, the Ge and C fractions scarcely changed and the B concentration (CB) increased proportionally. The C fraction increased proportionally with increasing CH3SiH3 partial pressures. These results can be explained by the modified Langmuir-type adsorption and reaction scheme. In B-doped Si1-x-yGexCy with y = 0.0054 or below, the carrier concentration was nearly equal to CB up to approximately 2×1020 cm-3 and was saturated at approximately 5×1020 cm-3, regardless of the Ge fraction. The B-doped Si1-x-yGexCy with high Ge and C fractions contained some electrically inactive B even at the lower CB region. Resistivity measurements show that the existence of C in the film enhances alloy scattering. The discrepancy between the observed lattice constant and the calculated value at the higher Ge and C fraction suggests that the B and C atoms exist at the interstitial site more preferentially.