http://www.eapsa.org/parents/hirschsprungs.htm has an excellent discussion of this rare condition. It's probably not what your 3 year old's problem is, but you should be aware of the remote possibility.
One of the things not made clear on the web sites I looked at prior to responding, is that this condition occurs with varying degrees of severity. Mild cases can be managed without surgery, although regular use of stool softeners and frequent cleansing enemas are required to maintain regularity and good health.
In such mild cases the parents may decide to defer any decision on surgery to a later date when the child is old enough to make the decision for him or herself.