Instead of 3-on-3, teams consist of four fighters. Three fight in sequence, while the fourth acts as a "Striker," jumping in to provide assist attacks at the cost of a "Strike Bomb".
One reason this rom is so sought after is the ease of accessing secrets. Even if the rom doesn't have them unlocked by default, use this cheat: Kof 99 Plus Rom
grants super armor, preventing the character from flinching when hit. Features of the "Plus" and Anniversary ROMs Instead of 3-on-3, teams consist of four fighters
Assuming you have legally obtained a .zip file labeled kof99p.zip or kf99plus.zip : Instead of 3-on-3