Bring a medium-size saucepan of salted water to a boil over medium heat. Add the couscous and cook until al dente, 7 to 8 minutes. Drain the couscous and set aside to cool. Meanwhile, in a small bowl, whisk together the lemon juice and zest with the olive oil to make a vinaigrette. Season with salt and pepper to taste.
In a large serving bowl, mix together the couscous, parsley, cilantro, mint, tomatoes, and scallions. Toss with the vinaigrette and season to taste. Allow it to sit for at least a half hour so the flavors can marry.