Based on your image, select A2:D10 and use this formula
=$D2>=LARGE($D$2:$D$10,3)
Select the range A2:D10 and enter this Conditional Formatting formula:
=Rank($D2,$D$2:$D$10)<=3
and choose your green formatting option
EDIT: Too slow, I guess (didn't see Fluff's response), but this show's you a different way to do it.