Wash 6 (8-ounce) canning jars, bands, and lids in hot soapy water. Rinse well; dry bands and lids. Keep jars submerged in hot water. Place a circular cooling rack in a large Dutch oven, and fill three-fourths full with water. Cover and bring to a boil over medium heat.
Heat a large cast-iron skillet over high heat. Add Fresno peppers, banana peppers, and bell peppers; cook, turning occasionally, until charred on all sides, about 5 minutes. Let cool enough to handle. Seed and dice Fresno and bell peppers; seed and slice banana peppers.
In a medium enamel-coated cast-iron Dutch oven, bring vinegar, pectin, and peppers to a boil over high heat, stirring frequently. Remove from heat; stir in sugar. Return to a boil; cook for 1 minute. Skim off any foam.
Remove jars from water. Using a ladle, divide jam among hot jars, leaving ΒΌ inch of headspace from top of rims. Using a damp cloth, clean rim and threads of jars. Add lids; screw on bands to fingertip tightness. Carefully place jars in Dutch oven; make sure water covers jars by 2 inches. Cover pot, and bring to a rolling boil; boil for 5 minutes. Remove from heat; uncover and let stand for 5 minutes. Using tongs, remove jars from Dutch oven, and let stand at room temperature until jars are completely cooled and have sealed. (Lids should not flex up and down when center is pressed.)
If jar does not seal, refrigerate up to 1 month. Refrigerate up to 2 weeks after opening jar.
Recipe by Southern Cast Iron at https://southerncastiron.com/charred-pepper-jelly/