Let us say that you are testing three cameras ...
Download the DISKBOOT.BIN from the SDM website for your cameras and replace the existing version on your bootable cards.
Power-up.
If the header at the top of the screen is red, you are setup for a 'left' camera.
The significance of this is that if the flash did fire it would only be at 1/16 normal power and undetectable in the image.
If the header is blue, the camera is set to 'right'.
In Stereo menu, set position to 'L' (for left).
In menu Stereo\Synchronisation, enable synch delay and set the user coarse and fine delays.
For now, set coarse to 10, 20 and 30 on the three cameras.
Exit menu mode.
Press the USB switch, screen will blank and blue LED will light.
Release the switch.
After one second first camera triggers, then second, then third.
If it does not work, I have forgotten something.