You are asking from a safety standpoint. The reason it is a problem to put a baby on a bed is because their head is relatively the biggest part of their body, and they have weak neck muscles. They can get themselves caught in the crack of the bed between the wall, and be unable to get their head out of it and suffocate. By the time they are 3, their body should have grown into their head and it shouldn't be a worry them getting caught. But if you want to be sure, ask their doctor. Call the office and ask if it should be a concern about them becoming trapped.
R.