Can this list comprehension be made more pythonic?
For integer division, use the // operator.  In Python 3, / will do floating-point division.

The task could be more succinctly accomplished using re.findall().

Also, the goal of the function is not obvious.  Pythonic code would include a docstring explaining its purpose.

import re

def split_string(s):
    """
    Splits a string into a list, with each element consisting of
    three characters.  Any incomplete group at the end is discarded,
    and then the last complete group is also discarded.
    """
    return re.findall(r'.{3}', s)[:-1]