The short answer is that you did it right.
Here are the two situations: three square pieces of pizza, each has side length S = 8.5 inches and one round pizza with diameter 16 inches (that makes the radius R = 8 inches).
The area of a square is S2, so the area of the three square pieces is 3S2. The area of a circle is πR2. So when you put your numbers in for R and S, get the numbers and comparison that you already worked out.
This has made me hungry!