Place the cucumber in a food processor fitted with the metal blade and pulse until coarsely chopped. Transfer the cucumber to a large bowl. Repeat the process with the red onion and transfer to the bowl with the cucumber.
Take half of the tomato and pulse in the food processor until chunky and add to the bowl.
Smash the garlic cloves and place in the food processor with the remaining tomato pieces, salt and pepper, vinegar and oil. Process until smooth.
Transfer mixture to the bowl with the cucumber and onion and stir to combine. Cover and refrigerate until cold.
Before serving, garnish with avocado. Can be made several days ahead.