Preheat the oven to 400°. In a large ovenproof skillet, heat the remaining 3 tablespoons of olive oil over moderately high heat. Remove the hens from the plastic bag and add them to the skillet, skin-side down. Cook until browned on the bottom, about 10 minutes. Using tongs, turn the Cornish hens skin side up. Transfer the skillet to the oven and roast the hens for 12 minutes, until cooked through.