Prep salad veggies. Slice the cucumbers; shave the carrot into ribbons with a vegetable peeler; and mince the onion.
Blend avocado, mayo, lemon juice, cucumber, herbs, salt, and pepper until smooth. Add more cucumbers a slice at a time, blending well each time, to thin out dressing if it's still too thick.
In a large bowl, toss the cucumbers, carrot ribbons, and minced onion with the herby avocado dressing.
Garnish with extra herbs if desired.
Keep the cucumber salad refrigerated until ready to eat.