<p>Some 3&nbsp;mm filaments seem to actually be 3&nbsp;mm - is there any way to shave off the excess and use it as 2.85&nbsp;mm? </p>

I would not recommend you to try and somehow re-size the filament, since even the smallest of irregularities and error in diameter occurring from such a process would ruin your prints with sporadic over and under extrusion. Rather, if you have the tools available, you could grind the filament into pellets, and use a filament extruder to make it anew with your desired diameter.
Alternatively, depending on your printer setup, you might very well extrude true 3.00 filament with your 2.85 mm filament printer. If you try to do that, make sure to:
Adjust filament diameter in your slicer
Check that your filament isn't getting squashed by the extruder wheel
Check that all mechanical parts actually can pass through your filament freely
I do not own a 2.85 mm printer myself, and therefore have not tried this procedure. There are, however,
several people
who seem to have done this successfully.