When the Constitution was being written, there were proposals that would have set up a plural executive.  For example, the New Jersey plan proposed such an executive.  I would argue that a plural executive would be much less decisive and powerful than a single executive as we now have.  That would be both the advantage and the disadvantage of such a system.

If all three executives had to agree on everything, it would be much harder to get anything done in the government.  The executives would surely have differences in opinion which, when combined with the differences within Congress, would end up making it very hard to pass any legislation.

People who want more efficient government would think this is a major drawback.  However, those who want the government to do less and be less intrusive would argue that such an executive would be a good addition to our system since that system is supposed to make it harder for the government to act and to, thereby, take away our freedoms.