Put a 4-quart stockpot over high heat and fill with 2 quarts of water. Season the water with 1/4 cup kosher salt and bring to a boil. Once the water comes to a boil, add the shrimp to the pot and immediately turn off the heat. Let the shrimp sit until just cooked through, about 3 minutes. Remove the shrimp from the water and spread out on a sheet pan to cool.
Once the shrimp are completely cooled, chop into 1/2-inch pieces and put in a medium-sized nonreactive bowl. Add the scallops, lemon, lime and orange juices and stir in the cucumber, red onion and chiles. Refrigerate for 1 hour.
Stir the tomatoes, avocado, chopped cilantro, and olive oil into the shrimp and scallop mixture. Allow it to sit at room temperature for 30 minutes, then season to taste with kosher salt.
To serve, divide the ceviche between 6 chilled martini glasses, garnish with cilantro leaves.