Class 3 is moderate-to-high chance of a fire. Burning permits (depending on your state law) usually are not allowed on Class 3 days. Class 3 usually means it is dry, high winds, and a mistaken fire...
12" is a good rule of thumb for 2-hour rating....but the 2006 IBC will give you a more accurate indication, plus industry web sites for concrete vendors will give you more accurate info.....