Apple Inc. sold more than a million units of its latest iPhone model in the first three days, making it the most successful model yet.

The iPhone 3G S went on sale Friday in the U.S. and seven other countries.

When Apple Inc. launched the previous model last year, it also sold one million units in the first three days, but that model launched simultaneously in 22 countries.

Piper Jaffray analyst Gene Munster had expected the Cupertino, Calif., company to sell...