an ohm of difference, make a difference?

I'll start out by saying this:  every amplifier is different.  While we know that Ohm's Law governs the way any amplifier will respond to a given load of resistance (ohms), and that the Law doesn't change no matter what brand or model of amplifier is used...

the way any given amplifier maintains quality at the different ohm loads does change.  That is, one amp (brand/model) may have a much more respectable quality of output at a lower ohm load than another amp....even if the ratings are similar at the standard 4 ohm measured rating.  While Ohm's Law describes the electrical reaction of an amplifier, the build quality of the amp is what makes the difference in how that amp responds to lower resistance, as far as sound quality goes.

So, maybe or maybe not, to the question of audible difference between 3 and 4 ohms.  I would think, as I'm not sure of this...that looking at the difference in manufacturer's specs between 4 ohm and 2 ohm would give a pretty good idea of the quality at 3 ohm.  So it is the amp with the closest of these two specs that will have the higher quality in between.

My take on the second question.  The vast majority of car audio users play at minimum load for the maximum power, as you know.  A few audiophiles, DYohn for example, play at the higher loads.  There must be a good enough reason to do this.  There again, it is everything to do with the quality of the music-reproducing equipment.  Everyday quality will likely not yield appreciable results.  The very high end stuff will show the difference. 

It is all about the work the amplifier has to do to produce current.  More resistance= less work, but less output.  Lower ohm load= more work= more current= more power...but less quality.  A better quality amp can work harder and produce more current and still have respectable specs.  And those specs get more and more respectable as the work it has to do is lessened.