Each device will have its own timebase and they won't match exactly. You will need to supply a shared timing signal to all of them via physical wiring. All the devices will need to either derive their sample clock from this signal (or possibly use directly as a sample clock).
I often sync devices controlled by the same PC without using a trigger, but your situation is different. It might call for a start trigger. I expect you probably have at least 2 separate PC's involved to support PCI, PXIe, and USB at the same time. The task sequencing I typically use may not be practical across 2 or more PC's, so it might be simpler to use triggering. (Note: you shouldn't have to share the trigger signal. Just make sure that the *source* of the shared sample timing signal doesn't start generating it until the trigger asserts.)
-Kevin P
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