Do you have any FX3 firmware samples that implement Composite and HID devices ?
No. At this point of time we don't have a plan to develop one.
I've seen few, I mean very few HID devices that need USB2.0 speed. Please let me know the reason behind this requirement.
We had composite device on FX2 that transfers data using slave FIFO and also has HID medical keyboard in the same firmware. It would be great to be able to implement the same things on FX3.
It should be doable, You'll have to modify the callback that handles USB setup requests to handle the HID requests as well and modify the endpoints used by the HID devices to handle the traffic. But we don't have plans for such an example right now.