Why was China broken into 3 fragments before the Mongols came to power?

Before Mongols came to power, there were 3 states, Liao, Song, and Jin. Liao and Jin used to be nomad’s tribes living in the northern prairie while Song controlled the central plain. Jin was originally called Nuzhen, which was under the reign of Liao, until it claimed independent in 1115. Since then, 3 powers formed and were against each other.