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Yes, you can certainly mix
Yes, you can certainly mix the berries. Here's an example of berry ratio: 8 ounces blueberries 4 ounces raspberries 8 ounces blackberries No, you don't have to use pectin. It is natural gelling agent found in ripe fruit. Some fruits has more pectin than others. Blackberries have high pectin levels. Strawberries have low levels so lemon juice is also used help the set. Many people find it difficult to make pure strawberry jam without adding pectin (which you can purchase in a liquid extract or in powdered form). Try it both ways and see what you prefer!