Combine first 3 ingredients in a large, heavy saucepan, and bring to a boil over medium heat, stirring occasionally. Cook for 2 minutes, stirring frequently with a whisk. Remove from heat; add butter, stirring until melted. Stir in molasses. Let stand for 15 minutes or until warm (100° to 110°).
Lightly spoon flours into dry measuring cups; level with a knife. Combine 3 cups all-purpose flour, whole wheat flour, rye flour, salt, and yeast in a large bowl. Add warm cornmeal mixture to flour mixture, stirring with a wooden spoon until combined.
Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 8 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel tacky).
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover the dough, and let rise in a warm place (85°), free from drafts, 1 1/2 hours or until doubled in size. (Press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down. Cover and let rest 5 minutes.
Divide dough in half. Working with 1 portion at a time (cover the remaining dough to keep from drying), roll each portion into a 14 x 7-inch rectangle on a lightly floured surface. Roll up each rectangle tightly, starting with a short edge, pressing firmly to eliminate air pockets; pinch seam and ends to seal. Place each roll, seam side down, in an 8 x 4-inch loaf pan coated with cooking spray.
Lightly coat the loaves with cooking spray; cover and let rise 1 hour or until doubled in size.
Preheat oven to 375°.
Bake at 375° for 40 minutes or until loaves are lightly browned and sound hollow when tapped. Remove the loaves from pans, and cool completely on wire racks.