The utilities will tell you that you only pay for what you use.
I know that determining the "fixed" and "variable" portions of the utilities can be done, but I don't recall how to do it and it probably isn't something that could be easily explained here. If we assume that the fixed and variable portions of the utilities are equal, then an equal share (1/6) of the fixed utilites would be fair. Start with a sixth and adjust it until you feel the amount is fair. Keep in mind that the current roommates shouldn't be paying anymore than they were before. This appraoch is very theoretical.
So here's a practical approach to your problem based on the same theory. Average all the previous month's utilities and take 2/3 of it to determine the average use by the remaining roommates. Subtract the current month's utilities by the previous amount. The difference belongs to the roommate that moved out and should average out over time, assuming constant utility rates, ceterus parabus in economics terms.

