Japan’s role in this game is to draw out the Ruskies unit they can be crushed, as a bonus to steal as many IPCs from the allies as humanly possible. There is nothing wrong with going all 3 routes, just don’t advance if it will be too costly. Typically though the allies have a hard time getting more units into India/China/North Russia though, other than using Russians of course which is difficult when Japan is beating on her. Going for Aus/Haw/New Zel is without a doubt worth it if the allies abandon the pacific, use an extra transport once you have ICs on the mainland pumping out the armor.
A good J1 purchase is 1 IC, 1-2 Trans, and/or 1 DD. You should always attack SZ 52 on J1, and Chi. If you can attack India and/or Bury as well you should do so, even if you will loose them on your next turn. Killing allied units is your job, you can get more units into India within a turn, the UK cannot. Also, it is good to kill the UK fleet if it runs to Egypt on J1 if you can. Draw the Russians away from the German front, then kills them. Typically Japan ends up taking Moscow around the time Berlin is ready to fall in the older version of this game.
As far as the US goes you have to decide, atlantic or pacific. If going atlantic liberating Africa as early as possible is a good idea (turn 1 or 2). Just be careful of the german air force. A good US1 buy would be 1 AC, as many DDs as needed to protect your transports, and then extra transports/units if you want. I would personally go 1 AC, 2 DDs, 1 Trans, 1 Arm if going KGF. Then move your inf from CUS over, your DD to SZ 10, and your BB/Trans to SZ 20. This gives you a nice sized force landing in Algeria on US2 with a nice sized fleet backing it up. If the germans purchased planes or subs, you may want an additional cruiser instead of the extra transport/Armor. Just try to keep the core of your navy as destroyers and aircraft carriers as possible, cruisers are extremely overrated for there shore bombard they rarely use. Getting units into Europe is all that matters.
If going pacific, or a split, with the US, good luck I havent tried that yet.