The 3 inf & 2 trn buy is my first buy as JApan in a standard or RR game.
After that, buy 1 trn each turn untill you have 6-7 trns.
You should be shipping a minimum of 6 inf and 1 arm to Man by J2 each turn. This ratio of 6:1 is good for infantry as fodder and arm to aid the ftr attack #s(ftrs are extra mobile tanks.) This is enough to chew off most of Asia unless UK/US reinforce or build an IC. Even with UK/US reinforcements you are helping Germany.
Follow DM & MB’s idea for Haw sz.
After Pearl you can move the Japanese fleet toward India(if not taken early due to Brit retreat) and/or Africa to support Germany’s holding it and to link up for mutual support. Don’t forget to send a trn or two with inf aboard along.
You may find the Japanese IC in Asia unnecessary. I often do. Sometimes I take those IPCs and build a trn and 2 inf to sail to Brazil. Maybe Hawaii or Alaska with additional support(bb or another trn and arm.)