1. Rinse the sushi rice under cold water until the water runs clear.
2. Cook the rice according to the package instructions. Once cooked, let it cool slightly.
3. In a small bowl, mix the rice vinegar, sugar, and salt until dissolved. Pour over the cooked rice and mix well.
4. Place a sheet of nori on the bamboo mat, shiny side down. Spread a thin layer of rice over the nori, leaving a 1-inch border at the top.
5. Arrange the cucumber, avocado, and fish slices in a line along the bottom edge of the rice.
6. Using the bamboo mat, roll the sushi tightly from the bottom, pressing gently to shape the roll.
7. Slice the roll into bite-sized pieces with a sharp knife. Repeat with remaining ingredients.