If you're familiar with Unplugged, you should be able to jump right into Rock Band 3's Expert modes on DS. The gameplay is identical: all the instruments and the vocals are charted at once, and you freely switch between them as you attempt to successfully play note phrases to keep each part "alive." It's Harmonix's classic Frequency game, but with rock music. Rock Band 3 for DS doesn't take advantage of the touch screen or the built-in mic -- nor does it require you to pay attention to both screens at once. The top screen displays your characters performing, while the bottom screen is the familiar note stream.
Nearly all of the features of Unplugged make a reappearance, but some are given new names. For example, "Band Survival Mode" is now called "Pro Mode," adopting the moniker given to the console game's biggest new feature (but you won't be plugging your MIDI instruments into the DS). Additionally, The Tour and Career modes have been enhanced in the some of the same ways as the console version: there are now "unique gameplay challenges" (objectives) that you'll need to achieve to progress.