In an effort to discover new thermoelectric materials, p-type Co2Ge3-xMxS3 (M = Zn, Cd, Bi; x = 0, 0.01, 0.03, 0.05) skutterudite-based chalcogenides are prepared using a traditional solid-state reaction. The samples are then compacted into pellets by hot pressing for thermoelectric characterization. A comparative investigation is performed by partial substitution of Ge sites with various dopants to assess the influence of substitution on the thermoelectric properties of Co2Ge3S3. Electrical and thermal transport properties, i.e., electrical conductivity, carrier concentration, Seebeck coefficient, and thermal conductivity, of the substituted Co2Ge3S3 samples are measured. Substitution has a moderate positive effect on the transport properties of these ternary skutterudites. All samples exhibit p-type behavior. A maximum power factor value of 10.3 μW cm−1 K−2 is obtained at approximately 518 K for Co2Ge2.99Zn0.01S3. Compared to Co2Ge3S3, the lattice thermal conductivity of substituted Co2Ge3S3 is substantially reduced due to the atomic disorder at the Ge sites, and a minimum lattice thermal conductivity value of 2.19 W m−1 K−1 is obtained at approximately 675 K for Co2Ge2.95Cd0.05S3. Compared with the referred Co2Ge3S3 sample, the Bi-substituted skutterudite obtained the 21% reduction of the lattice thermal conductivity and 38% enhancement of power factor, resulting in a peak ZT value of ~0.22 at 763 K of Co2Ge2.97Bi0.03S3.