You have to analyse and recreate the reasons why Germany and Japan decided to go to war with the USA.
In the case of Japan, it was being squeezed economically by the West, and decided to take control of things. It needed the oil of the DEI with it’s large supply of ORANGE TRIANGLES, and considered that the USN was the only force capable of stopping it getting what it wanted. So: wipe out this force with a surprise attack, in the knowledge that Germany and Italy were committed to joining in the battle, and that Russia was powerless to intervene.
The navy had been nagging Hitler to let the U boats loose on American shipping for months, so declaring war freed them to get on with a war they knew would come eventually BEFORE the Americans were fully mobilised, and were in any case now engaged in the Pacific, which Hitler wrongly assumed the US would give priority to.
I think the key is to place enough convoy zones in the Atlantic representing US aid to Britain that Germany simply HAS to intervene to stop these supplying the UK with income/units.
Although the Tripartite Pact was not signed until August, assume that the general principle stands: if one of the 3 main Axis powers becomes at war with the USA, they all declare war automatically.
Incidentally, the USA should in no way supply aid to the USSR until it is at war with Germany!
I don’t like the idea of American units “converting” to Soviet pieces; otherwise the game will just become another race to pile the largest stack of units into Russia.
The Tripartite Pact between Japan, Germany, and Italy, 1940
The Governments of Japan, Germany, and Italy consider it the prerequisite of a lasting peace that every nation in the world shall receive the space to which it is entitled. They have, therefore, decided to stand by and cooperate with one another in their efforts in the regions of Europe and Greater East Asia respectively. In doing this it is their prime purpose to establish and maintain a new order of things, calculated to promote the mutual prosperity and welfare of the peoples concerned. It is, furthermore, the desire of the three Governments to extend cooperation to nations in other spheres of the world that are inclined to direct their efforts along lines similar to their own for the purpose of realizing their ultimate object, world peace. Accordingly, the Governments of Japan, Germany and Italy have agreed as follows:
ARTICLE 1. Japan recognizes and respects the leadership of Germany and Italy in the establishment of a new order in Europe.
ARTICLE 2. Germany and Italy recognize and respect the leadership of Japan in the establishment of a new order in Greater East Asia.
ARTICLE 3. Japan, Germany, and Italy agree to cooperate in their efforts on aforesaid lines. They further undertake to assist one another with all political, economic and military means if one of the Contracting Powers is attacked by a Power at present not involved in the European War or in the Japanese-Chinese conflict.
ARTICLE 4. With a view to implementing the present pact, joint technical commissions, to be appointed by the respective Governments of Japan, Germany and Italy, will meet without delay.
ARTICLE 5. Japan, Germany and Italy affirm that the above agreement affects in no way the political status existing at present between each of the three Contracting Powers and Soviet Russia.
ARTICLE 6. The present pact shall become valid immediately upon signature and shall remain in force ten years from the date on which it becomes effective. In due time, before the expiration of said term, the High Contracting Parties shall, at the request of any one of them, enter into negotiations for its renewal.
http://www.ibiblio.org/pha/policy/1941/411211b.html