That is my preferred timing as well. J3 seems to be the best turn to begin the attack on the USA and UK/ANZAC. If for some reason the UK attacks before the USA is at war……all the better for Japan to begin its move to the money islands and India.
On R1 if they leave the stack of 18 infantry exposed it is a fatal mistake to not attack. Later in the game if the stack lives to retreat most of your ground forces and air force will be focused on the UK, USA and China…not the Russians. So by taking them out early you can force them to slip their forces…even if you are not driving to Moscow it still distracts them from focusing entirely on Germany.
Delaying an extra round also gives you another turn to take out China. I saw Japan ignore them in a previous game (J2 attack) and they took back almost all of China by round 6…it was not pretty, only a major IC held them back and by then the USA was knocking on Paris’s door. Germany was 2 rounds from taking Moscow…but that was too long as the USA got involved too early with the J2 attack.
J3 delays Japan in getting the extra cash, but it helps to position your fleets and troops for well focused and sustainable attacks on the allies. Plus Germany and Italy benifit greatly from that extra turn of breathing space…especially as the US tends to focus more cash on Europe.