In a large, deep skillet, heat 2 tablespoons of the olive oil. Add the squash, season lightly with salt and cook over high heat, stirring, until tender and lightly browned, about 8 minutes. Using a slotted spoon, transfer the squash to a plate. Add the remaining 1 tablespoon of olive oil to the skillet. Add the onion and sausage and cook over high heat, breaking up the sausage with a spoon, until lightly browned and cooked through. Return the squash to the skillet.