Blanch the shelled favas in boiling water for 1 minute. Drain, then rinse in cold water to stop the cooking; drain again. Pinch the favas out of their skins.
Preheat the oven to 350°. In an ovenproof skillet, heat 2 tablespoons of the olive oil over moderate heat. Add the favas, garlic, squash and zucchini; season with salt and pepper. Cook until just softened, 3 minutes. Transfer to a plate.
In a medium bowl, beat the eggs with the milk; season with salt and pepper. In the same skillet, heat the remaining 1 tablespoon of olive oil over moderately high heat. Pour in the eggs, then stir in the cooked vegetables, sorrel, peas, scallions and chives; season with salt and pepper. Cook the frittata over moderate heat until the edge is set, 2 minutes. Transfer the skillet to the oven and cook for 25 minutes, or until the frittata is set in the center and browned on top. Serve hot or at room temperature.
Make Ahead: The frittata can be refrigerated overnight. Bring to room temperature before serving.