Rinse couscous under cold water until water runs clear.
Bring the broth to a boil in a saucepan over medium heat. Add the couscous and cook according to the package directions. Drain well.
In the meantime, heat the oil in a sauté pan over medium heat. Sauté the squash, zucchini, and onion for approximately 5 minutes. Toss in the couscous and butter. Season with salt and pepper to taste before serving.