My FM2N with the B3 focusing screen is obviously brighter and snappier to focus than any other Nikon I have used, including the F3. A 3-series screen is probably what you have in your FM3 already.

Still, I grab the "darker" F2 boxes (all with B screens) far more often when heading out for a walk. I've never tried to quantify or even understand it, but they're "easier" to focus somehow.