There's really no set amount of time that a 3 year old child should poop.
How often a 3 year old child will poop depends on how much fiber the 3 year old child consumes and the more they eat the more they poop.
It's normal for a 3 year old to poop everyday and then some 3 year olds may only poop once or twice per week which is also normal.
As long as your 3 year old child pooping once per week then they are usually not constipated.
If the 3 year old goes longer than a week without pooping then I would say they are constipated and the 3 year old should see a doctor just to make sure everything is okay.
My kids when they were 3 years old would sometimes poop everyday and then other times they would go a week without pooping.
They were healthy and not constipated and the doctor said it was normal.