How to Make It
Whisk together bread flour, rye flour, whole-wheat flour, salt, and yeast in a large bowl. Stir together water and molasses in a medium bowl. Add to flour mixture; stir until flours are incorporated. (Dough will be very sticky.) Scrape dough into a large bowl coated with cooking spray. Cover and let rise in a warm (85°F) place free from drafts 3 hours.
Scrape dough onto a floured surface. Fold dough into thirds, like folding a letter. Rotate dough 180°F. Repeat folding and rotating procedure 2 times. Coat an 18-inch piece of parchment paper with cooking spray. Place paper, cooking spray side up, in a large bowl. Place dough, seam side down, in prepared bowl. Cover and let rise in a warm (85°F) place free from drafts 30 minutes.
Preheat oven to 450°F. Place a 4-quart Dutch oven in lower third of oven. (Do not remove Dutch oven while oven preheats.)
Remove Dutch oven from oven. Carefully place dough on parchment in hot Dutch oven. Cover and bake in preheated oven 30 minutes.
Uncover and bake until well browned and a thermometer inserted in center of bread reaches 195°F to 205°F, 5 to 10 minutes.
Transfer bread to a wire rack, and cool completely, about 1 hour.