Cook the Pasta Properly: Boil salted water and cook pasta until al dente. Drain and rinse under cold water, then let cool completely.
Prepare the Creamy Ranch Base: In a large bowl, combine ranch dressing, mayonnaise, and sour cream. Mix until smooth, then taste and adjust salt and pepper.
Chop and Add the Mix-Ins: Dice cucumber, halve cherry tomatoes, and finely chop onion. Add to the bowl with dressing and gently stir to coat.
Combine Everything Together: Add cooled pasta to the bowl and gently fold until evenly coated. Adjust consistency with more ranch if needed.
Chill Before Serving: Cover and refrigerate for at least 30 minutes. Stir before serving, loosening with ranch or milk if necessary.