On #1, I don't know the context of the question. I believe you are probably correct. He certainly believed we were strong enough to stop the 'spread" of communism (or Soviet influence) in the world.
#2 and #3 are correct. Chambers was right in there with McCarthy, looking for communists everywhere, looking for traitors, and sure they were everywhere. Hiss was convicted and served a jail term, but insisted for the rest of his life that he was a loyal American. Some of his views were, undoubtedly, unpopular. It's possible he got in trouble and falsely accused only because he held unpopular views, but we'll never know for sure.
1. Why does Truman believe that communism is not a danger to the United States?
A: Truman believes that communism is not a danger to the United States because he believes that the country is strong enough to resist and overcome all forces of communism.
2. Why does Whittaker Chambers believe that Communists are dangerous?
A: Chambers believes that Communists are dangerous because "disloyalty is a matter of principle with every member of the Communist Party."
3. How does Alger Hiss deny that he is a Communist?
A: Hiss denies that he is a Communist by claiming unflinching loyalty to the United States.
2 answers
(a) Why does Truman believe that communism is not a danger to the United States?