In a large pot over low heat, combine the strawberries and lemon juice. Cook, stirring gently and occasionally with a silicon spatula until half thawed, about 4 minutes.
Stir in the blueberries and raspberries and continue cooking, stirring gently and occasionally until the berries are completely thawed and have released a lot of juice, about 10 minutes.
In a small bowl, whisk together the sugar, corn starch, and salt. When the berries just start to simmer, stir in the sugar mixture and vanilla extract. Use a silicone spatula to fold the berries, stirring continuously, making sure to scrape the bottom and sides of the pot. When the berries have turned glassy and thickened, about 2 minutes, turn off the heat.
Pour the fruit into an 8x8, 7x10, or similar ceramic or glass baking dish. Allow the fruit to cool to room temperature before putting on the crisp topping, about 1 hour.
See crisp topping recipe (above) for baking instructions.