1.) overwrite the ambientpedmodelsets.meta in the LML Mod you have that has ambientpedmodelsets Active (Ambient Gang or Cowgirls Revisioned) 
2.) delete the ambientpedmodelsets.meta in lml>Ambient Cattle (you can safely delete the Ambient Cattle folder too if you are already using the merge)
3.) you still need the rest of the main files of Ambient Cattle mod in > lml\replace\update_4\x64\levels\rdr3\scenario\