If you are using wooden skewers, place them in a container with water to cover for 30 minutes. Preheat the grill to medium high.
For the dressing, in a small bowl or container combine 2 tablespoons olive oil, the lime zest and juice, orange juice, red pepper flakes, and salt and pepper. Set aside.
In a medium bowl, toss shrimp with the remaining tablespoon of oil and season with salt and pepper. Thread the shrimp onto skewers.
Carefully oil the grill rack. Grill the shrimp over medium heat until they turn pink and are just cooked through, about 4 minutes in all, turning the shrimp halfway during cooking. Let cool to room temperature and remove the shrimp from the skewers.
In a large shallow serving bowl, combine the tomatoes, onions, and half the cilantro. Add the half of the dressing and toss. Peel and half the avocados, remove the pit, and dice. Add the avocados and cooked shrimp to the bowl and toss gently to combine. Drizzle with the remaining dressing, and sprinkle with the remaining fresh cilantro.
Serve in the bowl, or serve over the arugula on individual plates.