Add three sum challenge (#108)

Co-authored-by: Silen Naihin <silen.naihin@gmail.com>

agbenchmark/challenges/code/d5/custom_python/test.py

@@ -0,0 +1,31 @@
+# mypy: ignore-errors
+from code import three_sum
+from typing import List
+
+
+def test_three_sum(nums: List[int], target: int, expected_result: List[int]) -> None:
+    result = three_sum(nums, target)
+    print(result)
+    assert (
+        result == expected_result
+    ), f"AssertionError: Expected the output to be {expected_result}"
+
+
+if __name__ == "__main__":
+    # test the trivial case with the first three numbers
+    nums = [2, 7, 11, 15]
+    target = 20
+    expected_result = [0, 1, 2]
+    test_three_sum(nums, target, expected_result)
+
+    # test for ability to use zero and the same number twice
+    nums = [2, 7, 0, 15, 12, 0]
+    target = 2
+    expected_result = [0, 2, 5]
+    test_three_sum(nums, target, expected_result)
+
+    # test for first and last index usage and negative numbers
+    nums = [-6, 7, 11, 4]
+    target = 9
+    expected_result = [0, 2, 3]
+    test_three_sum(nums, target, expected_result)