Place cucumbers in a colander and sprinkle with 1/2 teaspoon salt; toss to coat. Let stand 30 to 60 minutes. Pat cucumbers dry with paper towels. Transfer cucumbers to a medium bowl.
Chop 2 of the avocados; mash the remaining avocados. Add mashed and chopped avocados, green onions, cilantro, and lime juice to the cucumbers; stir to combine. Season to taste with additional salt and pepper.