Heat 2 Tablespoons olive oil in a large saucepan over medium-high heat. Once hot, add couscous and toast until fragrant, 5-6 minutes. Add 2 ½ cups chicken stock and a pinch of salt. Cover and simmer for 10 minutes.
To a large skillet, add 1 Tablespoon olive oil and tomatoes. Season with salt and pepper. Cook, stirring occasionally, until they become saucy, 5-6 minutes.
Add remaining chicken broth (1 cup) and fish sauce. Bring to a boil, then reduce heat to a simmer. Cook until the sauce is slightly thickened but still brothy, 3-5 minutes.
Off heat, stir in cooked couscous and lime juice and season with salt and pepper. Let sit a few minutes to allow broth to absorb. Season with additional salt and pepper and serve.