Area is 2 dimensional. It's just a cross section. It has nothing to do with length. Length is more for determining tuning.
For a 3" port, it's Pi * 1.5 squared. So actually for two it would be (3.14 * 1.5^2)2. That's closer to 14 sq in than the 28 that you stated.
You're confusing it with port volume
., which you need to find net enclosure volume, but doesn't directly affect tuning.