In a small bowl whisk together the lime juice, lemon juice, orange juice, ½ teaspoon salt, and garlic.
Place shrimp in a shallow baking dish or container and pour the citrus mixture over it. You want to make sure all of the shrimp is in the juice. Cover with plastic wrap and place in the refrigerator for 30 minutes. The shrimp is done when it is no longer translucent and has turned a pink and white color.
Add “cooked” shrimp and juice to a large bowl with the red onions, jalapenos, and tomatoes. Sprinkle with the remaining salt and toss together. Cover and put it back in the refrigerator for 30 minutes to let the flavors meld together.
When ready to serve peel and dice the avocado and add to the bowl along with the fresh cilantro. You can add a bit more salt if needed.