My contention is, in small spaces, efficiency is almost a non factor, granted temperature can be maintained at an acceptable level.
The end goal, is to reach a minimum density of photons produced, in your given area, regardless of the source.
Factoring Wattage consumed per SQ foot will get you in the ballpark.
3ft square, you're in need of a GENERAL ESTIMATE, of how much wattage you will need to CONSUME, to generate a sufficient amount of light, to get acceptable results...
IMO, getting into the 40w(ish) per square foot, will achieve this....so 120 Watts consumed, even at below average efficiency, should get it done....
For entry level LED growers, the sheer volume of options available is enough to overwhelm and confuse the majority....
Take a look at the thread titled, "Screw in Grow"..... for inspiration....