ELDERSBURG — It took twins Molly and Owen Pruitt only a few minutes to learn how to manifest a massive cube of hot obsidian suspended 100 feet in the air above a verdant valley. Then they added fruit.

Raspberry Pi has sold its three millionth unit, a little over two years after the low-cost educational computer now beloved by developers went on sale. The $35 Raspberry Pi Model B astonished when it ...