Minor bug

The easy variable has the word "Hard" in the text description, and vice versa:

easy   = tk.Button(intro, text = 'Hard
hard   = tk.Button(intro, text = 'Easy

I think it should be:

easy   = tk.Button(intro, text = 'Easy
hard   = tk.Button(intro, text = 'Hard

DRY

Lines like these are repeated in a couple places, with only the string changing, and also the string is the same for the 3 lines:

easy.config(state = 'disabled')
hard.config(state = 'disabled')
medium.config(state = 'disabled')

You can avoid the repetition by creating a function like:

def set_all_state(value):
    easy  .config(state = value)
    hard  .config(state = value)
    medium.config(state = value)

In the difficulty function, the expression len(word) appears several times. You can create a variable so that you only evaluate the expression once:

word_len = len(word)

Naming

The PEP 8 style guide recommends camel_case for function names. Change whenGuessed to when_guessed.

Layout

Move the functions to the top after the import lines. Having them in the middle of the code interrupts the natural flow of the code (from a human readability standpoint).

Documentation

Add a doctring at the top of the file to summarize the purpose of the code:

"""
Hangman game using tkinter GUI with 3 difficulty levels.
"""

There is a hard-coded directory path buried deep in the code:

file = open(r"C:\Users\d_j_d\Desktop\words.txt", 'r').read().split('\n')

It would be a good idea to mention this in the header documentation for the user.

Add docstrings frfor your functions as well:

def difficulty(minimum, maximum):
    """
    Set the difficulty level of the game.
    Input minimum: minimum length of the word
    Input maximum: maximum length of the word
    """