Apple shares were down $15.31, or 2.2 percent, at $684.79 in premarket trading.
The iPhone 5, the most eagerly awaited phone of the year, went on sale Friday in the U.S., Germany, France, Japan and five other countries.
When it launched the iPhone 4S a year ago, Apple sold 4 million in the first three days.
Apple could have sold more iPhones if it had more available: demand exceeded supply for most models. Topeka Capital Markets analyst Brian White said the phone was sold out at 80 to 85 percent of the U.S. Apple stores he and his team contacted Sunday evening, and the ones that were still available were mostly Sprint models.
Online delivery times have stretched to three to four weeks.
White had expected Apple to sell 6 to 6.5 million iPhone 5 units in the first three days.
The phone will go on sale in 22 more countries this Friday and in more than 100 countries by the end of the year.