For Playstation 3 and Xbox 360 you could use the same hints below:
Unlock Musha & Musha MK II Gundam
Musha Gundam - Challange "Trial of the Giants 4" must beat Trial of Giant 1-3 to unlock 4.
Musha MK II Gundam - Beat "Trials Come From the Past 9" Must beat 1-8 to unlock 9.
Unlock Knight Gundam
Knight Gundam Pilot - After unlocking the Suit, you should get a Special mission called "I am Knight Gundam!" beat it for Pilot.
Knight Gundam Suit - Beat "The Neverending Battle" in Chapter: For Reform.
Knight Gundam SP Equipment - Beat "The Neverending Battle" On hard then read message @ the Terminal.
Dynasty Warriors: Gundam 3 Game Info
Developer: Koei, Omega Force
Publisher: Namco Bandai
Platform : PS3, Xbox 360
Genre : Action role-playing game, Hack and slash
Release : JP December 16, 2010, NA June 28, 2011, EU July 1, 2011, AUS July 12, 2011