Consider an Electronic Control Unit (ECU) mounted in a vehicle. Let's say this ECU has Three CAN channels as follows:

CAN 1
CAN 2
CAN 3

Do these Three CAN channels have to be on different CAN networks? Another words, for example, CAN 1 and CAN 2 are never on the same CAN network?

It wouldn't make sense to connect CAN 1 and CAN 2 on the same network because why would ECU want to receive what it transmits? And why ECU want to receive identical information on CAN 1 and CAN 2?

 

Every CAN transmitter receives what it transmits. That is how it recognizes an ERROR frame. Since there is also a limit to the number of nodes on a CAN network, I'm guessing that it would make sense for the three CAN networks to be on physically different cable systems. The ECU is perfectly capable of bridging between the networks, if that is required.