In Bad Piggies's Ground Hog Day 1-2, you need to go left for the white box and cross the finish in under four seconds for the three stars—it appears impossible to it all at once. What's the trick?
It will require 2 separate runs.
For the first run, just build a cart with a blower on the left side. This headed straight for the goal will be enough for you to get the under-4-second star.
For the second run, build a cart with a blower on the right side. Then blow the cart with enough force so that it will explode against the TNT. The pig will roll down the hill to the finish. This should give you your remaining 2 stars.
I found a video walkthrough here.