Does 3x refer to the brim size?
The X refers to the quality of the felt. Wool is inferior material, and I don't own a wool hat. IMO if you care about what you're buying in a fedora or other hat, it won't be wool, it will be felt.
The X system of grading is variable from manufacturer to manufacturer, but more is better. Price goes up with felt quality.
Stetson says this on their website: The quality of the hat body used to make a hat is the main factor that determines the X's. In felt, it is determined by the percentage of furs used in making the hat body. We use beaver, mink, chinchilla and other animal fur to make bodies. The mixture of which furs we use determines the X's.
In my experience, decent felt hats start around 3X, especially hats from 40+ years back, which is mostly what I own. Ebay is still a decent source.
Brims are determined by the way the hat is made, although wider brims can be trimmed down (if they're not bound, as some better hats are). The the height of the crown is determined by manufacture. There is much good info at The Fedora Lounge. Articles | The Fedora Lounge