I do not believe the Dred Scott Decision had anything to do with the three-fifths provision. The people in the North actually wanted to count slaves as zero and the people in the South wanted to count them as one, but this was only for the purposes of deciding the apportionment of representation in the House of Representatives.
ML/NJ
The so-called "3/5ths compromise" had to do with counting population for the numbers of congressional representatives since (as it is today) the number of Representatives is based upon the population figures. The compromise was that 3 out every 5 slaves would be counted for population numbers thus entitling the southern states higher number of Representatives.
The 3/5ths compromise went back to the Articles of Confederation period and had to do with tax assessments to the states. The idea was that a slave worked less productively than a free person, so that five slaves would do the work of three free persons. The formula was then taken over in the Constitution for apportioning representatives (and also "direct taxes").