Preheat broiler or outdoor grill.
In a medium skillet over medium-high heat, add olive oil, garlic and seafood. Cook, stirring occasionally, until shrimp are completely pink and the calamari and scallops are opaque, about 3 to 5 minutes. Remove from heat, and let cool to room temperature.
In a medium bowl combine diced tomatoes, onion, avocado, and basil. Season with salt and pepper, and stir with a wooden spoon to combine all the ingredients. Add cooled seafood and a splash of olive oil, stirring to incorporate.
Toast the bread under the broiler or on the grill. Top each piece with a large mound of the seafood and tomato salad.