2 Samuel 24:13 (NKJV)
So Gad came to David and told him; and he said to him, "Shall seven years of famine come to you in your land? Or shall you flee three months before your enemies, while they pursue you? Or shall there be three days' plague in your land? Now consider and see what answer I should take back to Him who sent me."
1 Chronicles 21:11-12
So Gad went to David and said to him, "This is what the LORD says: 'Take your choice: three years of famine, three months of being swept away before your enemies, with their swords overtaking you, or three days of the sword of the LORD - days of plague in the land, with the angel of the LORD ravaging every part of Israel.' Now then, decide how I should answer the one who sent me."
This is a copyist error (see the translation footnotes). Presumably the correct number is three, since the other choices are threes.