I’d probably put the brisket on top since I would have probes coming out to monitor the temps. I’d think this would keep those out of the way of pulling racks out. Then ribs below and potatoes on the bottom.
And depending on where you are with the brisket temp, I would consider wrapping it, especially if it’s near or in the stall just to keep it from going backwards when starting to load the ribs and potatoes. Not sure if the extra mass or cold meat would do anything to the progress or not.
That’s just how I would do it with that cook. No other rhyme or reason. Just how I think I would do it.
Hope that helps.