The plain meaning is that He recognized the twelve hours of daylight to form a "day," and the corresponding twelve hours of darkness He called "night." Thus three days and three nights would be made up of six twelve-hour periods or 72 hours.
This is the same method He uses in the Old Testament—in fact, in the very first chapter of the Bible! "God called the light Day, and the darkness He called Night. So the evening and the morning were the first day. . . . So the evening and the morning were the second day. . . . So the evening and the morning were the third day" (Genesis 1:5, 8, 13). Here are three days and three nights so clearly defined anyone can understand.