3 ft shouldn’t be a big problem, but testing with another adapter can’t hurt.
Have you seen this:
Thoughts on optimizing gain
There is no one best gain setting but the above is a good guideline.
What is your gain set to?
You might have electrical noise in the area, sometimes the FA filter with the ProStick Plus just doesn’t cut it in regards to noise.
Personally i would recommend the rtl-sdr v3 dongle plus their LNA.
You could try shielding the dongle from the power supply you use. Maybe it’s generating noise.
As a test you could wrap the dongle it aluminium foil. Probably not good for the summer though in regards to heat, so not a permanent solution.
But for testing it should be fine.