Intensity Shuttle USB 3.0 with OBS and delay

Are they supported in OBS software?

If I hook up 3 HD cameras to 3 of these to my laptop (Asus G75VW), what would be the delay when switching from one to another in OBS?

OBS supports the shuttle, I use it myself.

I believe that in earlier topics it was stated that it's only possible to have 1 shuttle per system at a time. You probably need other hardware for that. If I'm wrong, please correct me.

Correct. You cannot have more than one Intensity Shuttle USB3 on a system - as the USB3 bandwidth does not support that much data throughput.