How many cups of self rising flour should I use instead three plain flour?

You can't directly substitute self-rising flour for plain flour in a 1:1 ratio. This is because self-rising flour already contains baking powder and salt, which plain flour doesn't.

Here's how to make the substitution:

For every 3 cups of plain flour:

* Use 2 1/4 cups of self-rising flour

* Omit any additional baking powder and salt that the recipe calls for.

Explanation:

* Self-rising flour typically contains 1 1/2 teaspoons of baking powder and 1/2 teaspoon of salt per cup.

* To account for this, you need to reduce the amount of self-rising flour used.

* You also need to remove any additional baking powder or salt that the recipe calls for to avoid over-leavening the baked goods.

Important Note: This substitution works best for recipes that rely on leavening from baking powder, like biscuits, muffins, and quick breads. For recipes that require a different type of leavening, like yeast breads, it is best to follow the recipe as written.