Remember your camera light meter still sees the entire world as an 18% grey. Set your camera first set your camera on manual, take a meter reading off something that is 18% grey ( your classic grey card or a piece of mid range coloured cloth should do ) keep the exposure and photograph something totally black under the same light and something totally white.
Then set the meter to P or Auto and photograph the 3 subjects using that under the same light.
Under P or Auto the camera will turn everything to the same shade of grey.
Also remember you can still adjust this in levels after the shot to a great extent
I must remember to take some photos to explain this
It is quite fascinating
"Photography to the amateur is recreation, to the professional it is work, and hard work too, no matter how pleasurable it may be."
( Total NIKON bigot )