Elephants are seldom part of Chinese history. Flip through 3000 years of wars and you will see countless mention of horses, but few about elephants. This is similarly reflected in Romance of the Three Kingdoms, where only 1 chapter out of 120 mention this beast.
In chapter 90, the King of the Southern Barbarians, Meng Huo 孟获, has been repeatedly humiliated by Zhu Ge Liang 诸葛亮 in battle. In his desperation to revenge, Meng Huo receives news of another barbarian king known as King Mu Lu 木鹿 and asks for his help to defeat Zhu Ge Liang. Mu Lu rides on a (you guessed it) white elephant. Though there was little mention of other elephants beside the King's ride, it can be understood that there are elephant warriors in the midst. One other barbarian king that appears in the same chapter is King Wu Tu Gu 兀突骨, also riding atop a white elephant. Both these kings were killed by the Shu army in the battles of Meng Huo vs Zhu Ge Liang.