China's three year plan would end in 1939. I can't remember the details, but the plans would make China self sufficient in small arms, and give Chiang mass production of 75mm artillery. It would also have completed various railroads linking the interior. It would also allow production of utility vehicles, radios, and other basic electronics and automotive parts. There were also plans for China to have oil drills for local oil production, and to convert coal to oil. They would also have industrial plants so that China could begin making valued added materials for their tungsten and antimony raw materials that they shipped in export. This would allow them to receive more money. China would still be fairly weak industrially, but it would now be self-sufficient in many essentials for war, and the critical war industries set up in the interior far from the coast. The added rail lines would greatly improve Chinese logistics.
1939 would also see completion of many of the initial plans of the German trained military. Chiang would have 60 trained divisions (as opposed to 8 when the war started in 1939). China's navy would be partially modernized with a dozen German made submarines, torpedo boats, and a German cruiser plus coastal defenses. Plus the "Chinese Hindenburg Line" of defenses around Nanking would be fully manned and defended for two years (it had just been completed prior to OTL's war). The air force was planned to have Me 109s and Stuka dive bombers. Chiang wasn't happy with the air force in 1937 and had already made plans to reform them, but the war interrupted them.
China would still be militarily inferior to Japan, but would be much better prepared for war. Likely the Chinese could quickly eliminate the Japanese presence in Shanghai, and adequately defend the Yangtze Delta so that the Japanese could not defend a bridgehead there. The major battles would be taking place in the North China Plain. Worse case scenario is that China holds the Yellow River, but loses the north (Hebei, possibly Shanxi). But there is pretty good chance they win and stop the Japanese from conquering the north. However, I don't think the Republic of China would be capable in 1939 of retaking Manchuria. They'd likely need several more three year plans before they could be ready.
Politically, three extra years of peace would greatly help Chiang as well. Most likely the CCP would be eliminated as an independent armed force. The party might still exist in quasi-legal status (as did other non-KMT parties), but they'd have no independent army. The 8th Route Army and New 4th Army may exist on paper, but they'd be integrated into the ROC order of battle with KMT officers and supplies. Chiang would also have consolidated his power in Guangzhou (brought into the central government's authority only in 1936) and Sichuan. With sixty trained and modern divisions, the Central Army would be completely superior to any warlord force, and he'd likely bring additional provinces into his control (meaning they pay taxes to Nanking, and Chiang loyalists govern the place). Most likely this give Chiang control of Shandong and Shanxi, perhaps even Guangxi. This would bring in even more tax revenue plus bring the independent warlord forces under his control.
There would be also other benefits. Late 1937 was going to be the long delayed elections to the National Assembly. They were supposed to happen in 1936, but were delayed because of practical measures. I believe they were to be held in October, but the war interrupted them. Holding of the elections would have transitioned the government from a "provisional" status to a more legal permanent one. Nothing would have really changed, but it would have improved Chiang's prestige and legitimacy internally and with the international community.
So three extra years would mean a lot to China. So if the war starts later in 1939, then Japan does not do anywhere near as well if full blown war happens. If it doesn't, then Chiang succeeds in eliminating the warlords and rules all of China in name by 1942. After a third three year plan is made, I think China will be ready to advance its claims to Manchuria again. Of course, with Germany involved in WWII, China would need some other power to provide the machine tools and other equipment for these other three year plans (most likely the US, although it would take some fancy financing as the US - unlike Nazi Germany - handles its foreign trade very differently).
If for some reason the three year plan is moved up in time so that it is completed in 1936, there are a lot of questions for that scenario. Is only the three year plan done, but none of the modernization of the armed forces or political gains? If not, then China is still better positioned, but its army is still manhandled early on. However, there is a good chance that China can win the Battle of Wuhan and hold onto the port of Canton. This would put China in a much better position to hold out for the remainder of the war. Many of the shortages and inflation China experienced in 1943-1945 would not happen, and Chiang would be in a much better position to win the Civil War.