(The three). One catastrophe, one locality, one day. These are Aristotle's rules for tragedy, and the French plays strictly follow them.
The French have added a fourth, one style. Hence comedy must not be mixed with tragedy. Addison's Cato is a good example. Unity of style is called the Unity of Uniformity. Shakespeare disregards all these canons.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Dramatic Unities from Fact Monster: