I don’t know to what extent Germany and Italy provided such assistance to each other, but a good study of the German-Japanese angle is “Reluctant Allies: German-Japanese Naval Relations in World War II,” by Hans-Joachim Krug, Yoichi Hirama, Axel Niestle, and Berthold J. Sander-Nagashima (published 2001). As I recall, there wasn’t much such assistance between Germany and Japan; this was partly due to the fact that they were on opposite sides of the world from each other, with enemy-controlled land and sea between them, and partly due to the fact that Germany and Japan were essentially fighting separate (though connected) conflicts, and ideologically were more like cousins than brothers. Japan did, as I recall, transport some strategic goods to and from Germany via long-range submarines a couple of times, but that was about it.
