@Jdoris - 4 legs would be more stable than 3, but I haven't had a problem with stability. I suspect going with 3 rods over 4 is the same '3 points make a plane' principle as using 3 footers under a component instead of 4; the least contact while still functional.
The rack in the link you provided does have a leg in back, so your previous observation may be true in this case.
The rack in the link you provided does have a leg in back, so your previous observation may be true in this case.