What words do three text files have in common?

Assignment

A word in a text is defined as the longest sequence of letters, where every character that is not a letter is considered a word boundary.

Write a function common_words that takes the locations of three text files (str). The function must return a set containing the words that occur in all three text files. The function must return a set containing the words that occur in all three text files. The function may not make a distinction between uppercase and lowercase letters, and the words in the set must be lowercase.

Example

In the following interactive session we assume the text files data_a.txt¹, data_b.txt² and data_c.txt³ to be located in the current directory.

>>> common_words('data_a.txt', 'data_b.txt', 'data_c.txt')
{'and', 'as', 'he'}