Preheat the oven to 400°F. Slice the bread into 1/2-inch thick slices. Lightly brush the bread slices with the 3 tablespoons of olive oil. Arrange the bread on a baking sheet and bake until lightly toasted, about 8 minutes. Remove from the oven and let cool before topping with the crab mixture.
In a large bowl, whisk together the lemon juice, 1/4 cup of olive oil, and salt and pepper to taste. Set aside.
Add the yellow pepper, avocado, shallot, and crabmeat to the lemon vinaigrette. Toss gently to avoid breaking up the crabmeat.
Top each bread slice with about a tablespoon of the crab mixture and arrange on a serving platter.