20 months old daughter has passed watery stools 3 times in last 6 hours? when should we get concerned and see a doctor?

When she feels weak: Diarrhea in small children is quite common and is mostly due to viruses, unless there is fever, mucus or blood is also seen and vomiting, in which case more evaluation may be needed. For now, give the child clear liquids by mouth, esp pedalyte, light soups, probiotics etc to keep her hydrated until this wears off in 2-3 days, Avoid dairy products, raw fruits and vegetables.

Gastroenteritis: Would immediately stop all dairy and start hydration with an electorlyte solution such as pedialyte or whatever is available in your country. If the diarrhea increases and your child becomes lethargic , then seek medical care. Another ominous sign would be bloody stools and high fever