You are correct that the last value in the set of three (e.g 680, 480, 30) is the one that sets the FPS value. I have not come across an instance where somebody has deliberately tried to change it to less than 30 though. It may be that 30 is an enforced minimum.
In Librealsense apparently, the FPS is set to 30 even if you specify another value. Users found that the reason for this was an instruction called Preset, and that to define their own custom FPS, they should use the Manual instruction instead of Preset.
I'm not sure that an equivalent command for doing so exists in the official RealSense SDK though.
This message was posted on behalf of Intel Corporation
Thank you for your reply and letting us know your results of testing different fps in HandMode and Face Mode. It seems that 30 fps is the minimum value you can use as explained by @MartyG. We looked at the SDK R3 documentation and didn’t find any mention on fps lower than 30.
Feel free to contact us again if help is needed.