Preheat the oven to 425°. Combine peaches, 1/4 cup sugar sugar, 1/4 cup brown sugar, cinnamon, lemon juice and cornstarch in a bowl. Toss gently to coat peaches. Pour all into a 2-quart baking dish. Bake for 10 minutes in preheated oven.
While peaches are baking, combine flour, 1/4 cup sugar, 1/4 cup brown sugar, baking powder, and salt in a large bowl. Blend in butter bits with a pastry blender, or your fingers, until mixtures resembles coarse crumbs. Stir in boiling water, just until combined.
Remove peaches from oven. Drop spoonfuls of batter over them. Dollops, rather than a uniform coating is best. (Otherwise, steam can't escape and topping is soggy).
Sprinkle 3 Tbsp sugar over top of batter and peaches. Bake about 30 minutes or until bubbling and lightly browned.