def decrypt_letter(letter):
    """ (str) -> str

    Precondition: len(letter) == 1 and letter.isupper()

    Return letter decrypted by shifting 3 places to the left.

    >>> decrypt_letter('Y')
    'V'
    """

    # Translate to a number.
    ord_diff = ord(letter) - ord('A')

    # Apply the left shift.
    new_char_ord = (ord_diff - 3) % 26

    # Convert back to a letter.
    return chr(new_char_ord + ord('A'))