Hi l4es ,
As AR0234 can't generate FSYNC signal, on OAK-D-LR we use GPIOs to generate that signal. I believe you could do the same; set all cameras to wait for signal, connect all cameras together and to some GPIO that you would then toggle (ideally PWM). Here's schematics for the OAK-D-LR:
Connecting all 3 AR0234 FSINs together:
To the SOM
Which is connected both to GPIO 46 and GPIO 49 (MX=RVC2, KB=RVC3, and we use MX). I'm not sure which GPIO is actually being used.