'Three Days' in the Bible

Meanwhile, David ordered Amasa, "Muster the army of Judah here within three days, and be here yourself!"

So Gad went to David and asked him, "Shall seven years of famine come to your land, or three months of reversals while you flee from your enemies as they pursue you, or three days of pestilence in your land? Decide right now what I am to answer to the one who sent me."