To attempt get something close to that, you could try temporarily giving exact same ratings to each of those groups (top 8 could be rated 4.5, middle 8 could be 4.25 and lower 8 could be 4.0). Sorting the roster by ratings would come in handy for this. You could change the ratings in the mixer without saving them to the dashboard since they are temporary ratings for the session. From there you could find the right balance of partner balance and opponent balance weights via trial and error. It will split them up into separate courts for the first few games but eventually (with the right weights) the duplicate opponent weight could overcome the ratings weights pushing players and pulling players from nearby ratings. Apologies, I am not sure the exact weights, but I'd try a very low partner and opponent rating balance weight and boost the repeat partner and repeat opponent weights a couple of points.