The Viper Mark III was original started to develped in respone to the Raider Mark IX.
The Viper Mark III enters service in the last years of the war and was hope to turn the tide back in the favor of the Colonial. However before the Viper Mark III enters full scale production the war ending. Because of this the Viper Mark III because just a minor note in the history of the Colonial unlike it more famous brother the Viper Mark II.
The Viper Mark III was design for more air combat than the Viper Mark II and because of this had a slightly difference body design.
After the war ending, work started on the Viper Mark IV which replace the Viper Mark III.
Images is taking for CanisD's Wolf Shipyards and was created by him.