The 1X is the elongated paddle, while the 2X is square-shaped. The 3X is a hybrid, landing in between those two. The 1X and 3X have the same weight and both come in 14 mm and 16 mm. They are both 16 ½" long.
The main differences are the handle and the shape. The 1X has a 5 ½" handle, while the 3X has a 5 ¼" handle. The 1X is 7 ¼" wide, while the 3X is wider at 7 ½".