you wont need it to be 3 foot high at all. the footprint is the main point, which would be 4 by 3. a motoro would prob be ok. i would go for a few of some of the smaller rays though, like yepezi, hystrix etc though and try breeding them
one thing though, you do want some height because you want enough gallons of water in there. they are messy and the more gallons the easier it is to control nitrates.
tru that.. well get crackin so i can buy one off you! If you take too long i may havta convert my biggest tank into a SW! JK how could i say no to that pattern
Make sure you stay with smaller species, as even a Motoro might get to big for this tank. Even though 4x3' is PLENTY of room for a stingray, a 24" ray with a 6-10" tail will basically be trapped in a small 'box'.. You would want to think about upgrading to 8x3'.
You could do Reticulata, Hystrix, or Scobina.. They all stay plenty small for life in that tank.