Rainbow Six 3 is a tactical shooter game DEVELOPED by UBI. The game is a first person shooter based on Tom Clancy's best selling novel by the same name. The game has two expansion packs, Athena Sword and Iron Wrath. The plot of Ravenshield is fully explained by Wikipedia (Tom Clancy's Rainbow Six 3). Basically it is all about an elite anti-terrorist group called Rainbow Six, a team bent on foiling the Neo-Facist terrorists doing damages on South American oil interests and European financial institutions.
The amazing RSASS for multiplayer servers. Features a highly accurate 6x scope and custom camouflages. Requires the Improved Custom Guns Mod (available here on ModDB). (This is version 2 - the rate of fire bug has been fixed.)