Your 3 year old pooping after every meal is usually due to the gastrocolic reflex which is normal.
The gastrocolic reflex is a natural and normal response after your 3 year old eats, which helps to clear your 3 year old's colon to make room for new food.
The gastrocolic reflex is also more sensitive in young children than in older children and adults and can result in a bowel movement and pooping shortly after the 3 year old or child eats.
If the poop that occurs after your 3 year old eats is consistently soft and normal, it's usually nothing to worry about.
Although if your 3 year old's poop is consistently loose or watery, it can be a sign of something else like toddler's diarrhea.
Also if the 3 year old has any other signs like abdominal pain, excessive gas, vomiting, fever or a poor appetite or if the 3 year old is not gaining weight properly then the 3 year old should see a pediatrician.
Also drinking too much juice or sports drinks and even drinking too much water can also result in loose and runny poop.
But in most cases the 3 year old pooping after every meal is normal and natural, especially if your 3 year old is eating enough fiber in their diet.
After we eat, our stomachs send signals to the colon to empty itself, which is a result of the gastrocolic reflex.
The gastrocolic reflex occurs in everyone of all ages, but the gastrocolic reflex is more pronounced in infants and young children, so pooping after meals including after every meal is common and natural.
Also the frequency of pooping can vary from child to child and needing to poop multiple times a day is normal.