Short Answer
The Kidnapper is the United States/Roosevelt. The Hooligan is Britain/Churchill. The Bully was the Soviet Union/Stalin.
For reference, this is the original passage from Chiang's diary:
聯合國中之四國，我為最弱，甚以弱者遇拐子、流氓與土霸為可危，也識知：人非自強，任何人亦不能為助。而國家之不求自強，則無論為敵為友，皆一汝為俎上之肉，可不戒懼？
Of the four members of the United Nations, we are the weakest; it is dangerous for the weak to be with a kidnapper, a hooligan and a bully. If a person does not strengthen themselves, no one else can help. And if a country does not strengthen itself, then friend or foe both see you as meat on a cutting board.
The Kidnapper
As @TomAu noted, this is a poor translation. The original Chinese phrase Chiang used, 拐子, actually has multiple meanings. There are two relevant definitions, neither of which is really captured by "kidnapper".
- a person with crippled legs
- a trickster who steal people or property through fraud.
It is apparent then, that here Chiang Kai-Shek meant Franklin Delano Roosevelt, President of the United States.
Note also that by 1943, Chiang had grown wary of the Western Allies. He was deeply dissatisfied with the Allies over their perceived European focus. Around February (a week before this entry), Chiang wrote of his suspicion that Roosevelt intended to use China to soak up the brunt of Japan's military might.
Such trickery would have made the United States a 拐子 in Chiang's eyes, at least at that particular juncture, in the sense of being a fraudulent scammer. Chiang later recycled this insult in a similar sense on Churchill exactly 13 months later. Outraged by what he perceived to be Churchill going back on the Atlantic Charter's promises, Chiang predicted in his 28 March 1944 diary that:
前後如此二人，英國拐騙手段暴露無遺，余斷此拐子末日必不遠矣
With such a two-faced act, England's deceitful tricks are completely exposed. I predict this kidnapper's end day cannot be far off.
The Hooligan
Hooligan is a pretty good translation. The Chinese word 流氓 is actually somewhat ill-defined, but indicates something along the lines of "a troublemaker without respect for laws or social order".
As the previous section shows, Chiang Kai-Shek harboured an intense dislike of Sir Winston Churchill and Britain because, from his perspective, the British Empire trampled over weaker nations without regard for right or wrong.
Chiang once vehemently wrote that "Today I learnt why Germany and Japan hated and must remove England and America. (乃知日德之所以必欲排除與痛恨英美之道矣)" And more specifically, when Britain backed out of a promise to fight in Burma and spoke of Tibet as an independent nation, a furious Chiang wrote that:
此誠帝國主義面目畢露，不僅為流氓市儈所不為，且亦為軸心倭寇所不齒
This exposes the true face of imperialism; not only do hooligans look down upon such acts, even the Axis and Japan will think this beneath them.
Hence, it appears that Chiang meant Great Britain and Churchill here.
The Bully
This is also an inadequate translation. The Chinese phrase of 土霸 is actually closer to a local ruffian. This locality distinction separates it from the 流氓 of the previous section.
While Chiang also thought of Churchill and the British Empire as scoundrels and bullies, he nonetheless recognised Britain as a global power with far flung territories. In contrast, although Russia is legitimately massive, it is concentrated in the frozen wastes of northern Eurasia. Moreover, the Soviet Union simply had much smaller of a global presence at the time.
In conclusion, Chiang is likely referring to Joseph Stalin and the Soviet Union here.
