We studied the effect of Ga3+and Sc3+ on Yb:YAG emission at cryogenic temperatures on three different mixed garnets namely Yb:YGAG, Yb:YSAG and Yb:YSGAG. The compositional tuning of these mixed garnets was achieved by preparing ceramic pellets by solid state reaction with different concentration of Ga3+, or Sc3+, or both. The incorporation of Sc3+ in Yb:YAG leads only to a limited spectral broadening. On the other hand, the incorporation of either Ga3+ or both Ga3+ and Sc3+ results in a significant spectral broadening. In the latter case, this inhomogeneous broadening is attributed to the mixed occupancy of both Ga3+ and Sc3+ in octahedral and tetragonal sites in the crystal lattice leading to a significant distortion in the structure.