Here's my take: Little Khorbasi is raised by Elves. The elves count to three on their fingers, by holding up all fingers except the thumb. When Khorbasi does the same, holding down his thumb, he ends up with four, which he calls "Three." Since Elves don't farm, use money or knit, this educational setback isn't noticed until he is told to bring back "three" children. Or at least that is the excuse he prepares for his tribemates.