Good Luck. Remember, all of these will tend to quiet down once they get the coating of slime.
I have three holes in my overflow as well, but I use them all for drainage. Two returns are run up the back out of my Ampmaster 4750. I have quite a bit more flow than you have going on here.
I use my three overflow bulkheads as follows: One (2") is the setup I described above at about 1" below the bottom of the overflow teeth and currently takes all of the flow. The second (1.5") is a Durso a little higher. The third (1.5") is just an open pipe at the very top of the overflow for an extreme emergency. It goes straight down and the pipe terminates 6" above the water level in the sump so that it is extremely loud if it ever gets to that point.
I don't know if this matters, but my drain has a constant slope before dumping into my socks.