At 3 years old the 3 year old child should weigh around 30 to 31 lbs which is a healthy weight and anything under 30 lbs for a 3 year old may be underweight.
My 3 year old son weighed 30 lbs when he was 3 and he was healthy but I also had another 3 year old that weighed 35 lbs because he was a bit more chunky than his brother at the same age.
As long as they are 30 lbs in weight at least at 3 years old they should be healthy.
At 4 years old they should weigh at least 34 lbs.
The average height of a 3 year old is around 3 feet tall however some may be a bit shorter and some may be a bit taller.
It all depends as nobody is the same height or weight at the same ages.