Look, I’m not sure if this is the best way to do it, but I tested it here and it works:
You can create a group with the data type “number” inside the RG #1 Services cell (e.g., “rg service number”). Set the group’s data source to the current cell’s index. Then, inside your RG #2 Service Task Groups, you can reference the group “rg service number: number,” and it will return the cell number from RG #1 Service. Do the same from RG #2 to RG #3.