This question was raised at that time, when Japan had not declared war on the United States. At that time, the Soviet-German War was a period of devastating blows by the Germans. There were two voices in Japan at that time. One is to go to the north, just like you said, stab the Soviet army in the back. The second is to invade Southeast Asia southward. The Indian Ocean is the interest of Britain and the United States. This is bound to offend the United States, and Japan has fully considered it. It's hard to tell the difference between the two voices at once. But the war situation at that time affected this decision. At that time, the Soviet Union was not fully prepared, and it was said that it was deliberately retreating. Anyway, the Soviet Union was losing ground at that time. Japan didn't think it was necessary to help its allies, and there was no need to spend manpower and material resources to occupy Siberian oil fields at that time. Japan is a country short of resources, while Southeast Asia is rich in expansion resources. So, in the end, the voice of the south defeated the north.

Later, Japan attacked Pearl Harbor.

History is thus frozen, and history cannot be assumed, if it can be assumed. Then, what is certain is that, at least, without the Soviet Union among the allies, history will be rewritten.

No, the spheres of influence were already divided when the three countries formed an alliance. The whole Soviet Union and Europe and America belong to Germany, Italy is in charge of Africa and Japan is in charge of Asia-Pacific.