Mountain Lion was released last week Thursday. Apple is charging $20 for the operating system update through the Mac App Store, making it a very accessible update for many people. In fact, in its first four days on the App Store, Mountain Lion was downloaded over three million times, making it the fastest-selling and most successful OS X launch in Apple’s history.

Apple senior vice president of Worldwide Marketing Phil Schiller:

“Just a year after the incredibly successful introduction of Lion, customers have downloaded Mountain Lion over three million times in just four days, making it our most successful release ever.”

These numbers fall in line with what web tracking firm Chitika reported last week. According to the firm, Mountain Lion was already installed on 3%  (2.11 million) of compatible Macs in its first 48 hours.

It should be noted, however, that in Apple’s case, downloads do not equal purchases or installations. For instance, I’ve only sent Apple $20, but already have it downloaded and installed on two of my family’s three Macs. On the other hand, one download could theoretically be used on more than one Mac if you made a bootable USB drive or DVD.

Still, three million separate downloads is a great number for Mountain Lion. Have you installed it on any of your Macs yet?