It is effectively like increasing the course rating by 3. So, if you shot an adjusted gross score of 90, and the Course Rating was 70, your usual gross differential would be 20, and if the slop was 130, then your Score Differential would be 17.4. With a PCC of 3, shooting a 90 would give you an effective "gross differential" of 17 rather than 20 and a score differential of 14.8. The 14.8 showing your 90 was a better score in the tough conditions than the 17.4 score diff when you shot 90 in average conditions. The full Score Diff equation is:

Score Diff = (113 / Slope) x (Adjusted Gross Score - Course Rating - PCC)