Three, a 3G-only carrier in Australia has announced substantial price cuts on many devices and service plans.

The new discount is set at $20 monthly credit on the existing plan. A two year contract now costs  $59 a month, which includes $350 of messaging and calls, unlimited BlackBerry data, and many other sops.

Clearly, with the launch of iPhone 3 G and of the App Store just hours away, competitors are gearing up for the onslaught, and devising ways to retain market share.

Rogers Communications in Canada and and Vodafone in Australia, among others, have a lesson to learn in this story.

A quick aside: Three is the carrier that did not get a deal from Apple to sell the iPhone. They’ve been asking their customers to petition Apple to allow Three to sell the iPhone. Read more about the petition.