How would the thinkers of the Enlightenment regard the ideologies of socialism,communism, and liberalism?
First, there was a great deal of diversity in Enlightenment thought, especially between France and England. Generally, though, the Enlightenment is seen as the birthplace of what is sometimes called classical liberalism. Enlightenment thinkers generally supported religious freedom or at least tolerance, freedoms of the press and speech, rights of the accused, and in constitutional government.
They did not generally support many ideas that would crystallize as socialism in the nineteenth century, but it is important to remember that the eighteenth century was largely pre-industrial, and that socialism emerged in response to the Industrial Revolution and the inequalities it generated. Many Enlightenment thinkers believed that inequalities made republican government impossible, and while few advocated government ownership of the means of production, some (Jean-Jacques Rousseau being the most notable) did argue that governments needed to preserve material equality to the greatest extent possible.
As for communism, some of its theoretical underpinnings (the dialectic and the materialistic view of history) might have been broadly acceptable to some Enlightenment thinkers, but again, it was the product of a very different Europe, one which bore the stamp of both the Industrial Revolution and the French Revolution.
In general, thinkers of the Enlightenment would have approved of liberalism and would not have liked communism. They might have approved of socialism, depending on the exact nature of a given socialist system.
The whole idea of liberalism emerged from the Enlightenment. Enlightenment thinkers were very much dedicated to the idea of the rights of the individual. It is for this reason that they would have embraced liberalism with its strong emphasis on those rights.
Communism would have been distasteful to them because of its lack of regard for individual rights. The idea of a system that guaranteed neither economic nor personal rights would not have met with their approval. Communism, with its insistence on the inevitable march of history towards a communist system, would have seemed just as illogical to them as a system that insisted on the divine right of kings. Both would seem to be based on faith and not on reason.
If a socialist system protected people's rights, it might have been acceptable to Enlightenment thinkers. They might have liked its emphasis on the equality of all people. However, it would have been important for it to be a democratic form of socialism that guaranteed political and economic rights.
Because the Enlightenment—the time period roughly spanning the 1700s—was based on the ideals of personal liberty, reason, and tolerance of others, Enlightenment thinkers would have favored liberalism far more than socialism or communism. Liberalism, after all, is a political and philosophical concept that is grounded in the lessons of the Enlightenment.
However, putting all Enlightenment thinkers into one box is impossible and may invite error. Prominent scholars in the Enlightenment were running with the momentum that started with the Renaissance and developing new and widely varied ideas about nearly everything.
For example, Thomas More, a Renaissance humanist writer and thinker who was a forefather to the Enlightenment Era and a big part of its development, wrote Utopia, which described a perfect society. In this fictional society, private property was prohibited, leading More to be praised as a communist by Karl Marx and hailed by the USSR as one of its leading thinkers.