We report on spectroscopic investigations of the mixed sesquioxide laser materials Yb:LuScO3, Yb:YScO3, and Yb:(Lu,Y,Sc)2O3 as well as mode-locked thin disk laser experiments with Yb:LuScO3 and Yb:(Lu,Y,Sc)2O3. The disordered crystal structures of these materials result in significantly broader emission spectra than for the pure sesquioxides Yb:Sc2O3, Yb:Y2O3, and Yb:Lu2O3 providing a high potential for ultrashort pulse generation. In SESAM mode-locked thin disk laser experiments, pulse durations of around 100 fs could be obtained exploiting >70 % of the gain bandwidth which is to the best of our knowledge the optimum obtained so far for the mode-locked lasers in this setup.