Thanks so much, I got it working now.
It may be easier to just solder a short piece of wire onto the right sides of the shunt resistors, and use those as inputs.
You still need to cut the path that runs between the right sides of these resistors. (so that's two cuts in total, no more)
By the way, there's no need to make a new connection between the left sides of the shunt resistors and the input connectors CH1/CH2/CH3; they're already connected.