Both the Communists and Warlords were a significant distraction for the Nationalists during the war against the Japanese, however the Nationalists did not fight all three at the same time.

The Nationalists actively avoided engaging with the Japanese until 1937 (the invasion of Manchuria occurred in 1931), with the intention of focusing their efforts to defeat the Communists. In 1937 a Manchurian warlord - Zhang Xueliang, "The Young Marshall" - kidnapped Nationalist leader Chiang Kaishek and forced him to enter into a truce with the Communists, and to form a united front against the Japanese.

As to the Warlords, the Warlord Period is usually considered to have ended in the late 20s after a Nationalist campaign to unite the country - the Northern Expedition. Parts of China did continue to be under the control of ostensibly allied warlords, over whom the Nationalists could exert limited influence. Quite limited actually, the Communists famously and repeatedly exploited the loose relationship between Nationalists and the Warlords to survive the Long March.

Even during the Sino-Japanese war and despite the truce, Nationalist attention seems to have been divided between the Communists and the Japanese. US General Stillwell frequently expressed frustration that the Nationalists did not appear committed to the war, and that Chiang Kaishek was more interested in preserving his forces for the renewal of hostilities with the Communists.

As to how they fought, the Nationalists were forced into a conventional war against the Japanese with inferior forces. They were unable to commit the necessary resources to resist the Japanese occupation of large cities, but were able to effectively resist over-extended Japanese forces in China's interior. The Communists in contrast fought a guerrilla war, avoiding pitched battles and occupying territory after Japanese forces had moved on.