Have on hand 3 heatproof ramekins or custard cups (16-ounce capacity each). Butter them generously.
Bring a large kettle of water to a boil.
In a mixing bowl, stir together the rye flour, cornmeal, whole-wheat flour, baking soda, and salt.
In another bowl, mix together the maple syrup and buttermilk. Stir the maple mixture into the flour mixture until smooth. Fold in the currants.
Divide the batter among the ramekins or cups. Butter 3 pieces of foil. Cover the ramekins or cups with foil, buttered side down.
Set the ramekins on the bottom of a large wide pot. Pour the boiling water around them until it reaches halfway up the sides. Bring the water back to a simmer over low heat. Cover the pot, and gently steam for 2 hours or until a skewer inserted into the middle of the breads comes out clean. With tongs, carefully remove them from the pan, set them on a wire rack, and let them cool.