No, you don't have to use all three types of berries to make berry lemonade! As I mentioned in the Ingredient substitution ideas section, you could replace one type of berry with an equal proportion of another, for example, using 12 ounces of blackberries instead of 6 ounces of raspberries and 6 ounces of blackberries. Or, you could even just use one type of berry, as long as you have the same total amount of berries called for in the recipe (20 ounces).