If you want more than 1 video input you need to mix the signals before they go into the Cadet III
3x Cadet III won’t work. The sync generator can only clock to 1 sync signal. 3x Cadet III would mean 3x sync signals.
The RGB input mod works with 1 (RGB) input signal that is connected to 3x Cadet modules. The sync is the same, shared across the video inputs.
The sync busboard is also intended for 1 sync source (which is the master).
it has 10x 14pin headers for output + 1 or 2 RCA outputs. (the slaves)
You can look at a video modular system like this too. (in a normal, non-glitch operation method)
There is 1 master sync, and all other modules get synced by this.
If you use a Cadet III, this will become the Master instead of the Cadet I and all other modules that use the sync signal adapt to this. (like VCO’s, Ramps, RGB output etc.)
I hope this helps with your questions.