On upper gastrointestinal series, the reverse 3 sign refers to thickening of the duodenal folds with puckering at the site of the ampulla of Vater. This is due to edema, both from duodenal inflammatory changes, or indirect changes involving the pancreatic head. It's classically associated with pancreatic head adenocarcinoma, but can also be seen with acute pancreatitis and duodenal carcinoma.
Gastrointestinal Radiology: The Requisites (2nd ed). p 85
Clin Imaging. 1991 Oct-Dec;15(4):283-5.