Yes. To award multiple winners, simply return to your contest and add more positions to your contest. Click on the "upgrade" button below your contest title. Upgrade popup will open where you can add more positions. Once added, make the payment for the additional designs.
You can buy as many winning designs as you like a single contest. The prices for 2nd and 3rd designs are 75% and 50% of the cost of the 1st design. But in the fast track contests, the prices for 2nd and 3rd designs are equivalent to the 1st design.