Referring to my earlier drawing, if you connect one of the common terminals from Rachio A to another common terminal on Rachio B using one cable, the two are now bridged. If you proceed to connect another common terminal on Rachio B to a free common terminal on Rachio C, all three of your controllers will now be bridged, with Rachio B acting as a pass-through between Rachio A and Rachio C. You do not need to connect Rachio A to Rachio C as it would create a loop and introduce interference into the system. Note that you only used one cable to connect any two of your controllers.
Between the three of your controllers, you should have 10 common terminals. You would need four of them to bridge your controllers as described above, leaving you with six available terminals to connect your field commons. Where you make these field connections (which of the 6 available terminals you use), does not matter.
Alternately you can simply bundle all of the commons together (four from the field and one common from each of your three controllers, for the total of 7 wires) and connect them all with one wire nut. In this case you should only use one common connection from each of your controllers, leaving 7 common terminals left unused, which is OK for this configuration.
Remember to only connect / bridge commons together, if you try to drive one of the zones with more than one controller (thereby making a second point of contact between individual controllers), you would potentially create a circuit and damage your controllers.
You can share commons, but sharing zones / valves / power is a recipe for trouble.