Most people avoid reflectors nowadays after a few instances of them melting and almost causing a fire. If your in a tent the tent will act as a reflector but they may still have some use in an openspace. I know Timber got rid of reflectors due to this. Also the reflectors, with the added hanging height they require, dont really add intensity on the cannopy, just hanging height. If you have a really large array of cobs in some sort of square you could add reflectors on the outermost cobs if youre in an open space to not lose light to the walls but if its a smaller light in a tent you dont really need them, just lower the light.
Has for placement/wattage for cobs arranged in some sort of square pattern (2x2 or as in your case 3x2 which can be described as 2 2x2 where the centre 2 cobs are shared between 2 squares):
In a square 4 cob config each cob is going to throw 1/4 of its light within the square (as it sits in a corner). So between the 4 cobs you will have the equivalent of one cobs wattage. Figure out what kind of intensity you want to be able to use per square foot (consensus around here seems to be 30-35w up to 40w maxing out). So take the desired intensity of one of your cobs and figure out how many squarefeet it would cover on its own. It will be the same as the square you draw between your 4 cobs, adjust the size/spacing to desired wattage per squarefoot. You can also work yourself to a desired wattage if you already know your desired spacing.
F ex say you want 40w per squarefoot and use your 40cm spacing: 0.4m*0.4m*11(approx of how many squarefeet to a m2)=1.76 squarefeet within your square. Multiply by 40w and you have your required intensity per cob at 40cm spacing: 1.76*40=70ish w per cob.
You can also reverse the equation to work out coverage area if you know how hard you will run your cobs.
Tldr: dont use reflectors unless as see above, dont put a cob in the middle of your cob square