My Kitchen Aid 600 was struggling and getting very warm just mixing two classic loaves. I found that by splitting the oil and honey between the oatmeal and the dough (half the oil and honey in the oatmeal and remaining half of oil and honey in the mixing bowl with the preferment and flour) it fairly easily mixes three loaves. Letting the dough sit for a few minutes before adding the oatmeal also seems to help.