Rinse the sushi rice under cold water until the water runs clear.
Cook the rice according to package instructions.
In a small saucepan, combine rice vinegar, sugar, and salt; heat until dissolved.
Once the rice is cooked, let it cool slightly, then mix in the vinegar mixture.
Place a bamboo sushi mat on a flat surface and cover it with plastic wrap.
Lay a sheet of nori on the mat, shiny side down.
Wet your hands and spread a thin layer of rice over the nori, leaving a 1-inch border at the top.
Arrange imitation crab, avocado slices, and cucumber along the bottom edge of the rice.
Using the mat, carefully roll the sushi away from you, applying gentle pressure.
Seal the edge of the nori with a little water and slice the roll into bite-sized pieces.