Here are a couple of threads I did recently testing out a party that gave pretty good detail. Great cooker. So easy to use. One thread shows ribs in there. 3 slabs would be tight on 1 rack. I cooked 3 slabs and used 2 racks. Not sure how it would affect airflow if you put all 3 on 1.
As for bringing the smoker up to temp it took about an hour to get to 225 with the water pan in which to me isn't long. I put the diverter in after it came to temp to help get the water steaming. I don't know if keeping both out would help. There are many other more experienced backwoods folks out there, but I bet it won't give you that much of a jump start if any. It gets going pretty quick as it is.