More specifically, did the Japanese fight in the European Theater, or the Germans and Italians in the Pacific? Did they just provide economic support or were they more hands on?
The Germans did have some submarines operating against Allied shipping from Japanese bases around Indonesia (Operation Monsoon). However, these operations could easily be seen as an extension of German submarine warfare, which was already operating in the South Atlantic and Indian oceans, rather than any direct support of Japan.
There was also some flow of technology from Germany to Japan. The Ki-61 fighter aircraft used a license built copy of the same German Daimler-Benz engine that powered the Messerschmitt Bf-109. There was other avionics and aircraft technology assistance as well, but I can't recall them off the top of my ahead. I'm not informed enough to comment about much outside of naval operations and aircraft.