Whispersync is Amazon's name for the connection between their devices and the Kindle servers. This started out as a private cell phone network on the Kindle 1/2/DX, but can be over the Internet (e.g. WiFi) on the Kindle 3 and on all Kindle software (Desktop, iPhone, etcetera).
Most Kindle ebooks can be authorized for 6 devices simultaneously, but each Kindle ebook file is locked to one device. Here, your Kindle 3 and K4Mac are each "devices". So you can read your Kindle ebooks on your Mac if you want, but in any case the needed ebook files will be downloaded to your K3 when it arrives (via either WiFi or AT&T cell phone network).
Unlike the Sony, you can buy from the Kindle Store with any web browser. The ebooks then get queued up to be downloaded to (say) your K3.
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