The 3 brackets are mounted between the towers and the top plate to create a rigid frame and a square angle between the plate and towers.
I printed these on a well tuned prusa i2 to ensure that the bracket is exactly square.
A m4 nut holder is added to insert m4 nuts into the 20mm extrusions. I had no t-nuts around, but this works just as well.
Mount the bracket to the tower using the m4 20mm extrusion nut holder and 2 m4 nuts. The brackets is then attached to the tower using 2 m4x16 screws with a ring. A 4.5x30 wood screw can be used to attach the part of the bracket pointing towards the center to the top plate