I have 5 above 10 and 1 at 9, and I regard myself as a mid-level DIII level team with borrowed spot in DII.
Based on experience, I would agree with Jelme - 6 above 10 is great for a long cup run, but a painful towards the end of the season and a short-lived playoff run.
For DIII, personally, I think you need at least 2 at prolific/sensational or above and the third at at least prominent. I am not saying it is always the case, but at DIII income levels, I would guess that most teams can only support 2 good outside scorers on their payroll. Just a guess based on gut feel, and I do not have much experience, but if I had to make a trade-off because on salary constraints, that is what I would do. Instead of a higher OD on the 3rd player onwards, I would instead invest in a wondrous/marvelous rebounder, because good D that result in missed shots is nice, but if no one is there to collect the rebounds, there is not much point. I would rather give up a couple of 3-pointers every game.
If you can afford to buy or train and keep 3 guys above 15, that would be perfect, but you probably will not be a d.III team for long. :-)