What are three different factors which contributed to the outbreak of the French Revolution, and which is the most important?

There were many factors that contributed to the French Revolution, which was a very complicated event, both in terms of causes and effects. Let us look at a few of the contributing factors.

First, France under Louis XVI was a society divided into three social and legal orders, or "estates." The First Estate encompassed the Catholic clergy, the Second included the titled nobility, and the Third was made up of everyone else, from desperately poor peasants and urban workers to the comfortable and educated bourgeoisie (lawyers, merchants, and businessmen). The First Estate and, for the most part, the Second was exempt from taxation. The estates had their origins in French feudal society, and were established by law and the support of the king. The problem was that this system was hopelessly out of date. Not only did it make it difficult to collect tax revenue, but it did not accurately reflect the realities of French society, where merchants and businessmen were increasing in wealth. While it was possible to buy one's way into the nobility by purchasing a royal appointment, many bourgeoisie came to see the nobles and clergy as useless parasites, and questioned why the legal and social system favored them. These educated men, along with many reformers among the nobility, would become the leaders of the Revolution's early stage.

Second, the 1780s saw a terrible fiscal crisis. This stemmed in large part from the antiquated tax system alluded to in the previous paragraph. What brought on the crisis, though, was the enormous expense of supporting the royal court and bureaucracy, and especially French participation in the war of the American Revolution. Several royal ministers attempted to reform the system, but they were consistently blocked in their efforts by the nobility, who controlled the French courts. Things were made even worse by a series of bad harvests in the 1780s, which not only led to near-famine conditions in the cities, but difficulty in collecting taxes from landowners who had little produce to sell. So the fiscal crisis was intertwined with an economic crisis that affected the majority of Frenchmen.

Finally, another contributing factor, albeit one debated by historians, was the spread of Enlightenment ideals. It is difficult to measure the effects of the ideas in the same way one can measure, say, bread shortages, but there is no doubt that this philosophical movement tended to erode the foundations of royal absolutism. Many of the educated leaders of the Third Estate were deeply read in Enlightenment texts like Voltaire's Philosophical Dictionary, Diderot's Encyclopedia, The Social Contract by Jean-Jacques Rousseau, and The Spirit of the Laws by the Baron de Montesquieu. These works, and many others, amounted to a sort of syllabus for liberal reformers in the late eighteenth century, and many of the ideas they contained--divided government, religious tolerance, penal reform--made their way into the constitutional monarchy established in the first phase of the revolution. Rousseau, by far the most radical in his views, was idolized by many of the Jacobins who initiated the Reign of Terror in 1794. 

It is a matter of opinion which of these is the most important. Scholars of the French Revolution differ dramatically on this question, but most today tend to emphasize the role of ordinary Frenchmen on the Revolution, an interpretation that would tend to make the economic crisis more important than others.