Grado's spot on with all the eggs in one basket under LED.. LED is a technology where your canopy needs to be almost perfectly level in order for all plants to get the desired effect from just one light. This is why you see a lot of LED growers with multiple lights. For 3x5 (15sqft).. you'd want to go with two lights, two pretty nice lights on top of that.. honestly, if you can handle the heat - you're better off with 1kW HPS in that size area..
No "600W" LED is covering a 6x6 efficiently, I'm just not buying that.. will you get light hitting all 36 sq ft? Sure.. will anything grow outside of the core 16sq ft? Probably not..
EDIT:
Any way to handle the heat? Your canopy is pretty level, go with 2 9 spot or 3 6 spot lights IMO.