Heat a saucepan over low/medium heat.

Stir 2 cups of blackberries, sugar, lemon juice, 3 tablespoons water into pan and bring to a boil.

Let boil for 3-4 minutes, then add remaining 1 cup of blackberries. Stir.

Mix 4 tablespoons water and corn starch together. Add to blackberries and stir.

Continue cooking until sauce thickens.

Remove from heat and either serve warm over pancakes, french toast, etc., or cool and use for toppings on cheesecakes, crepes, etc.