There is only 1 jump in the game I can think of that is big enough to get 3 seconds of air time. It's at Gallaxhar's Spaceship just before the finish line ( take the middle path ). Save up a Turbo power-up just to be sure and use it as you are going for the ramp. Make sure you're on 150cc.
You will also be able to get these 2 if you haven't done so already