The temp/dewpoint 3degree spread is a holdover from the first Jet era..The DC8/9 used this as an antiicing guide(pratt witneys).The vis moisture below plus 8 was used at this time.After the 'Palmair'crash at Washington the FAA went to a straight Vis.moisture below plus ten,although I see in some of the business jets manuals plus 8 is still a reference..
Cheers
