Invader III Fighters were a class of star fighter used by the Venomian Army during the Lylat Wars. These fighters were a vast improvement over the previous series, being highly maneuverable, fast and well armored. Invader IIIs could go up against even Arwings and put up a competent showing.
Invader IIIs were first seen in the Battle of Sector Z, where they ambushed the Star Fox Team. As part of a plan to cripple Great Fox, the fighters escorted six Copperhead missiles to be used against the capital ship. However, despite their advancements the Invader IIIs could not defeat Star Fox or protect the Copperheads to prevent them from being intercepted. Many of the fighters were destroyed in that battle.
Earlier within the war, pilot Katt Monroe had stolen an Invader III prototype from the Venomian Army and heavily modified it as her personal star fighter to use throughout the conflict.
References
NINTENDO POWER Starfox 64: Official Nintendo Player's Guide. Redmond, WA, Nintendo of America. 1997.