Good news is that your setup does not feature a master valve or any kind of sensor that I can see, thus you will not have to deal with an isolator or any sort of relay setup.
I see that you have at least four common lines. It is possible that you will be able to segregate your setup into four separate groups, but you will end up bridging at least one group between two different Rachios. This should not be a problem since voltage potential needs two points of contact for current to flow, and having only commons interconnected between the Rachios will not cause an issue.
If you are considering Gen 3 controllers with their flow meter capabilities. @jsore has outlined (link) a way to make this work with three additional master valves, since each of the new master valves would be connected to an individual Rachio, there should not be a need for isolators.
Have you considered how you wish to setup your schedule? Are there any watering restrictions where you live?