How did the rule of japan shift from the rule of the daimyo to the rule of the three great commanders?

The Shogunate began with the Yoritomo Minamoto. During his time, the samurai manage to thwart the Mongol invasion. Still the next shogunates were not strong and the Emperor still exerted influence. Power began to shift from daimyos to the three unifiers Nobunaga, Hideyoshi and Tokugawa. From these Japan was now starting to unite and the Shogun became the military ruler of Japan.