Fill a large pot with heavily-salted water and bring to a boil. Add barley and lentils, stir, and cook until al dente, about 20 minutes. During the last 2 minutes of cooking, add green beans and boil until knife tender.
Meanwhile, combine garlic, balsamic, lemon juice, salt, thyme, and black pepper in a large bowl. While constantly whisking, slowly add oil until incorporated.
Once barley and beans are tender, drain thoroughly and add to vinaigrette. Toss to combine, then add remaining ingredients. Stir gently and taste; adding more salt and pepper if necessary. Serve warm, room temperature, or cold.