It's OK about the questions
About the infeed, I will set pick load in the system that R1 must pick 33% second one must pick 50% and the third robot must pick 100%
In my imagination, it will split the load into three equal parts.
About the outfeed, we will control the speed of the outfeed conveyor which is carrying the boxes, this will let us set required amount of boxes.
About the PLC, this is the easy way, but I'm afraid of the communication delays, which can cause inaccuracy.
For your imagine it will be around 180 pcs per min. So in total it's around 60 for each robot, and accuracy can't exceed 1mm.