Hi @ugurdemirezen,
Without information on the camera you’re using or the code you’ve written it’s difficult to help.
By “when rov runs” do you mean when it gets powered on, or when the vehicle is armed?
Is this a limitation on your end? The .companion.rc file that’s used for setup when the ROV is turned on doesn’t have any file type requirements (it runs various python and shell files without issues).
What is the “it” that you did? Companion 0 isn’t expected to run on Raspberry Pi 4 by default, so I’m unsure how you tried something with Companion 0 that apparently worked on Raspberry Pi 4 but not Raspberry Pi 3.
Assuming your extra camera has a H264-encoded output option, you might want to try the approach I detailed here, which allows setting up multiple streams with only relatively minor modifications to the existing Companion video streaming code.