In a large mixing bowl, combine the wheat flour, rye flour, cornmeal, baking soda, and salt. Mix to combine.
Make a well in the center and add the buttermilk, molasses, brown sugar, and raisins. Stir until everything is just combined.
Lightly grease 4 wide mouthed, straight-sided, pint canning jars. Grease the lids as well. Divide the batter among the jars, filling about 2/3 the way up.
Place the lids on the jars, tightening only just enough so that the lids don't fall off.
Transfer the jars to the slow cooker. Fill the slow cooker with enough water to reach the batter line in the jars.
Cook on high until the batter passes the toothpick test, about 2 1/2 hours.
Remove the jars from the slow cooker and let rest until cool enough to handle. Remove the lids, then shake the jars to remove the loafs. Let them finish cooling on a wire rack.