My 3yr old has a swollen gland on the back of his neck. It has been large for a long time none of his doctors seem to be concerned. Could it be seriou?

Get a diagnosis. You should ask your doctors for a diagnosis of the cause of the swollen gland. Cannot say if it is serious or not serious until there is a diagnosis.
Possibly. Most of the time swollen glands in the back of a child's neck are not serious. They may be signs of an active or healing infection anywhere in the head or neck. Rarely, they can be a sign of cancer. It is important to stay in touch with your primary care physician to have the gland re-checked. If it persists, your doctor may choose to do further testing.