@Ben-0 assuming is the same PSU powering all three boards?
Assuming it is, then almost certainly there is a wiring problem. Here's a quick test you can do. The 3HC has a 4-pin termination jumper block. With power off, use a multimeter to measure the resistance between the two outer pins. It should be 60 ohms because there are 120 ohm termination resistors on both the main board and the tool board. If it reads 120 ohms then it's likely there is a bad connection between the 3HC and the 1LC. If it's 60 ohms then double check that CANL and CANH are not swapped between the 3HC and the 1LC.