Jim,
You are overthinking the issue. Best way to do think this through, is to not go over whatever the lowest load capacity is, in your case on one wheel your max load is 3200#. So If you are at 3100 on one side (wheel) then you are still under max load capacity. Now as for the axle, yes that is a combination of both wheel/tires even though their capacity is now over 6000#, you are limited to 6000# axle rating. So once again the lowest capacity rules. I don't think you will find anything that states official figures on this, but will state not to go over load capacity. Hope this helps. Clear as mud?
Don