Add tests, remove main in ThreeSumProblem

[Hardvan]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Uncyclopedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.95%. Comparing base (1dfa2e5) to head (335598e).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5781      +/-   ##
============================================
+ Coverage     65.59%   65.95%   +0.36%     
- Complexity     4369     4385      +16     
============================================
  Files           601      601              
  Lines         16795    16780      -15     
  Branches       3227     3226       -1     
============================================
+ Hits          11017    11068      +51     
+ Misses         5333     5266      -67     
- Partials        445      446       +1     

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