I was tested once for celiac disease and it came back negative. In three years could that have changed?

Check with you Doc. Discuss with your doctor why your were tested for celiac disease. As with any test, the degree of suspicion for a particular disease helps understand the results. If there was a very high index of suspicion for celiac disease and the test was negative it may need to be repeated or further testing may need to done like a biopsy of your small intestine.
Yes. Celiac disease is an IgA (antibody) mediated disease that can be tested through blood work. But a definitive diagnosis is best obtained through a biopsy sample taken during an endoscopic GI procedure. Hope this helps.
Simple answer=yes. There is the day, month or year before you get any problem. With a problem like CD, you can be born with a tendency toward developing the disease, but you are not born with it. At some point, you experience the event or events that trigger a change and begin to develop antibodies to gluten based proteins. These then begin to attack gut tissues. So yes this can change in three years.