In a blender combine avocado, yogurt, mayonnaise, lime juice, garlic, and salt. Blend until very smooth, adding cool water as needed to reach a thick but pourable consistency (¼-½ cup) and scraping down sides of blender occasionally.
Add dill, chives, and parsley and blend until herbs are finely chopped but not completely pureed.
Pour ranch dressing into a serving bowl and serve on a platter with carrots, celery, lettuce, and pita chips for dipping.
