I don't know about the American response to nationalism in these countries. I rather think the nationalist movements in these countries came in the 19th century; at least the unifications of the German states and of the Italian states came in the 19th century. The unification of Japan occurred in the 16th century.
I rather think your question was supposed to be about America's response to the rise of imperialism in Japan, Italy, and Germany during the 1930s. This America did take note of.
You were right to limit your question to Japan, Germany, and Italy; Franco of Spain does not belong to that category.
About the same time that America conquered the Philippines in southeastern Asia, Japan conquered Korea and Formosa in northeastern Asia. Observers then said that the next great war was likely to be between Japan and USA, but america had no national policy to prepare for such a war; a few naval officers may have considered alternative strategies against Japan in case of war, but this was just their professionalism at work, not a national policy.
It is probable that Hitler wanted to conquor the whole world; Japan wanted to conquor eastern Asia from Burma to eastern Russia, and the near-byIndies and Philippines, and the central Pacific Islands. In 1937, Japan invaded China; in 1939, Germany invaded Poland. At the beginning of 1939, President Roosevelt and his advisers started preparing the USA for war. USA soon started transferring its old arms and war ships to Britain to use in its war with Germany. At first the American people wanted to stay out of the war, but by 1941, a majority of Americans supported Roosevelt's preparations for war, while still hoping America's participation could be naval, air support, and manufacturing, not troops on the ground. In December 1941, Japan attacked Pearl Harbor and the Philippines. USA declared war on Germany, Japan, and Italy. Britain and USA agreed that Germany had to be defeated first, then Japan.
For the most part, the American public sought to remain rather isolationist in its perception of what was happening in Europe. The refrain of "it's Europe problem" as well as the challenges in dealing with life during the Great Depression, crippling economic and social challenges in America, really precluded much in way of interest in what was happening in Europe. President Roosevelt was fairly concerned with what was happening, forging diplomatic alliances with the Soviet Union and stressing to England its support. Yet, there was a significant inertia regarding United States involvement in the affairs of Europe. Hitler, Mussolini, Franco, and Stalin were not widely perceived as the threatening and menacing forces that they are seen now, as the presence of the moment gripped American psyche, preventing it to step outside of this temporal condition and be moved to action on the inclination of how bad things would be.
I am pretty sure that there must be some answer in your book that you are supposed to be coming up with.
In general, the US didn't really respond much to this. The US did not really care all that much about nationalism in those countries. It was only once the countries started to become very aggressive that the US cared.
Even once the countries started to try to take more land for themselves, the US did not really respond. It was not until things really got bad in the late '30s and early '40s that the US started to put pressure on them to stop.