to simplify geometry in modeler you can use qemLOSS3 or Simplify Mesh plugins. they works similar - just select proper layer, open plugin page, set percentage of simplification and watch for result. usually percentage for LOD1/LOD2/LOD3 is about 75/50/30 but it can be more (sometimes model became bunch of unrecognizable polygons) or less. visually you can make it like this - original model is perfect, LOD1 is OK, LOD2 is ugly, LOD3 looks like model from mid 80's game.