input: Write a python function to join three strings in pairs and return a list.
  assert strJoin("a", "b", "c") == ["ab", "bc", "ac"]
  assert strJoin("hello", "world", "!") == ["helloworld", "world!", "hello!"]
  assert strJoin("123", "456", "789") == ["123456", "456789", "123789"]
output: def strJoin(s1, s2, s3):
      return [s1 + s2, s2 + s3, s1 + s3]