My son has a hydrocele in his groin dr said to wait and watch he is 3 years old does it have a chance of going away by itself and is it normal that its in his groin it gets smaller or bigger. he had a ultrasound done its not always there?

There is the potential for it to disappear in time, but there may be a chance he would develop a hernia on that side years or decades later. You don't want to rush to surgery on something that poses no risk. It can be watched.

There is a fine sac communicating with the abdominal cavity which normally closes at time of birth. sometimes it closes late and can some fluid can be seen in it. Your doc's advice is sound. It may close. If not it can easily be repaired later.