A 3 year old should be able to concentrate for 12 to 15 minutes.
Concentration improves overtime with a child's age and the older the child gets the more they are usually able to concentrate.
Children usually are able to concentrate for a period of 3 to 5 minutes per the child's age.
So a 2 year old child should be able to concentrate 6 to 7 minutes at a time and then they lose concentration or interest in what they were concentrating on.
As for a 3 year old the 3 year old should be able to concentrate for 12 to 15 minutes.
And for a child of 5 years old they should also be able to concentrate for 15 to 20 minutes at a time depending on the child.
Every child is different and some concentrate for less time than others which is normal.