Apple has just announced that downloads of its Mountain Lion operating system hit three million in its first four days, making it what the company claims to be the most successful OS X release in Apple’s history.

Launched last Wednesday, Mountain Lion is a $19.99 download from the Mac App Store.

That doesn’t mean Apple has taken 3 million payments of $19.99, though. One payment allows a customer to download the OS to multiple machines tied to the same Mac App Store account. Also, some users took advantage of a program that allows recently purchased Macs to grab a the update for free. That scheme didn’t go entirely to plan at first though, with some customers reporting ‘Code Has Been Used’ errors when trying to redeem their free download.

By way of comparison, last year, Apple announced it had served one million downloads of the previous iteration, OS X Lion, in 24 hours.

Mountain Lion ushers in a raft of improvements to OS X, including an iOS-style Notification Center, AirPlay Mirroring and bolstered iCloud support. Read our full review for our verdict on Apple’s latest OS.

Image credit: USFWS Mountain Prairie