Combine flour, milk, water, eggs, salt, and 3 tablespoons butter in a blender. Blend until mixture is perfectly smooth, dislodging any flour that sticks to the sides with a rubber spatula. Let batter sit at least 15 minutes at room temperature, or up to overnight in the refrigerator.
Heat a 6-inch nonstick skillet, or crêpe pan, over medium-high heat. Lightly brush pan with melted butter. Pour about ¼ cup batter into the center of the hot pan, swirling it quickly to cover the bottom of the pan with a light coating of batter. Cook 45–60 seconds until the underside of the crêpe is a light golden brown throughout. Loosen by running a spatula around the edges, then gently flip and cook an additional 20–30 seconds on the underside (this spotty side is the side you fill—the more evenly brown side is the side presented). Repeat with the remaining batter, brushing the pan lightly with butter between crêpes.
As the crêpes are done, stack them on a wire rack to prevent sticking; serve warm (see filling ideas below). Store unused crêpes stacked in an air-tight container in the refrigerator for up to 3 days, or freeze for several weeks.