Firstly Hitler invaded in June of 1941, not 1942.

Italy did declare war on the Soviets along side Germany, sending an entire army of roughly 235,000 soldiers to assist the Germans.

On the point of Japan, they faced a critical decision of whether to strike north against the Soviets or south against the Western Allies' colonial holdings. Basically it was a fight between the Japanese Army and the Navy. The Army wanted to strike the Soviets and take Siberia and the Navy wanted to attack into the Pacific and take the rick resources of the South East Pacific. Eventually the Navy won out with the Yamamoto's plan to knock out the US Navy at Pearl Harbor looking like the best opportunity to move. And the rest, as they say, is history. After they became involved in the Pacific Front of the war in addition to the ground wars in China and Burma, the Japanese simply couldn't spare the forces to attack the Soviets and open yet another front.