In a small saucepan over medium heat, combine rice vinegar, sugar and salt. Heat mixture until sugar dissolves. Remove from heat and let cool.
Wash rice and place in a saucepan with water. Let soak for 30 minutes.
Drain rice and transfer into a heavy pot with 4 cups of water over medium-high heat until it boils. Reduce heat, set to low and cover for 15 minutes.
Turn off heat and let rice rest covered for an additional 15 minutes. Transfer into a large bowl, and sprinkle the cooled rice vinegar mixture over the rice, lightly mixing. Add enough to coat the rice, but not make it damp.
Spread the rice on top of a large sheet of aluminum foil and let cool.
Wash and peel cucumber. Cut cucumber into long slender strips. Cut the avocados into long slender strips, removing the pit and peel. Sprinkle avocado with lemon juice to help retain its color.
Place a sheet of plastic wrap on top of the bamboo mat before placing roasted seaweed on top. Spread a thin layer of the sushi rice over 3/4 of the seaweed sheets, leaving an inch of the sheet exposed at each end.
Take flatbreads and put them into a food processor so that they are crumb-like. Sprinkle on top of the rice and add another sheet of plastic wrap. Turn over the rice sheet so that the seaweed sheet is on top. Arrange the avocado, cucumber and crab meat in the center of the rice. Gently start rolling and finally cut the roll into smaller pieces.