Bring 2 cups water and quinoa to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until quinoa is tender and water has been absorbed, 15 to 20 minutes.
Bring 1 1/2 cup water and couscous to a boil in a separate saucepan. Reduce heat to medium-low, cover, and simmer until couscous is tender yet firm to the bite, about 10 minutes.
Bring 3 cups water and farro to a boil in a separate saucepan. Reduce heat to medium-low, cover, and simmer until farro is tender and water has been absorbed, about 25 minutes.
Combine quinoa, couscous, farro, cucumber, red onion, orange bell pepper, and yellow squash together in a bowl.
Whisk olive oil, lemon juice, and salt together in a bowl. Pour dressing over grain-vegetable mixture; toss to coat. Sprinkle feta cheese over salad. Refrigerate until chilled, at least 2 hours.