问题描述:
Write a function to merge three dictionaries into a single expression.

测试用例:
['assert merge_dictionaries_three({ "R": "Red", "B": "Black", "P": "Pink" }, { "G": "Green", "W": "White" },{ "O": "Orange", "W": "White", "B": "Black" })=={\'B\': \'Black\', \'R\': \'Red\', \'P\': \'Pink\', \'G\': \'Green\', \'W\': \'White\', \'O\': \'Orange\'}', 'assert merge_dictionaries_three({ "R": "Red", "B": "Black", "P": "Pink" }, { "G": "Green", "W": "White" },{"L":"lavender","B":"Blue"})=={\'W\': \'White\', \'P\': \'Pink\', \'B\': \'Black\', \'R\': \'Red\', \'G\': \'Green\', \'L\': \'lavender\'}', 'assert merge_dictionaries_three({ "R": "Red", "B": "Black", "P": "Pink" },{"L":"lavender","B":"Blue"},{ "G": "Green", "W": "White" })=={\'B\': \'Black\', \'P\': \'Pink\', \'R\': \'Red\', \'G\': \'Green\', \'L\': \'lavender\', \'W\': \'White\'}']

生成的代码:
def merge_dictionaries_three(dict1, dict2, dict3):
    return {**dict1, **dict2, **dict3}