What were the three religious practices found in Mecca before Muhammad's revelations?

The religious practices found in Mecca before the rise of Islam were of the sort that would typically be considered pagan. There were three deities who were most central to the Arabic practices at the time, sometimes referred to as the Daughters of God: The Goddess, al-Lat; The Mighty, or al-Uzzah; and Manat, a goddess of destiny. There were also many more minor deities beyond this. The Kaaba, which stands at the center of the great mosque at Mecca, was said to have been surrounded by 360 idols. These divine beings were not fully personified, but rather were represented symbolically, with worship directed, for instance, at large stones, or the morning sun. There were other beliefs in beings that may be understood as supernatural or divine as well, such as Jinn, or "the subtle ones."

The two main religions that influenced the development of Islam and which existed in Mecca in Muhammad's day were Judaism and Christianity. They were both monotheistic faiths. However, a large number of polytheistic faiths also flourished in the area, and polytheistic followers would make a pilgrimage each year to a stone building in Mecca called the Kaaba, which was filled with different deities. The Kaaba helped make Mecca the religious, cultural, and diplomatic center in the region even prior to the ascent of Islam.
Because it is monotheistic, Islam shares many similarities with Judaism and Christianity and accepts the religious prophets of those two faiths as its own prophets, claiming descent from Moses's son Ishmael and treating Jesus as an honored prophet, although not the son of God.
Islam is believed to have addressed barriers that kept people with a desire for monotheism from converting to either Judaism or Christianity. For example, it was not as exclusive a faith as Judaism and did not require circumcision. Further, it eradicated the confusion that Christianity presented with its trinity of three co-equal faces of God, which looked to many people very much like polytheism. Islam became a religion easy to convert to, and its monotheism was, from the start, clear-cut.
Muhammad made a vast number of converts to Islam in a short time period and, as polytheism died out, turned Mecca into a Muslim pilgrimage destination.

Three religious practices found in Mecca before Muhammad's revelations included the following:
Polytheism/Paganism: This was very widely practiced in Mecca before Muhammad began preaching. Polytheism in Mecca revolved around idolatry, with numerous idols being worshipped. Every tribe and house had their own idols, and the Kabah in Mecca was full of them. The Arabs at the time believed that the idols would bring them nearer to God and mediate with him for their sake. Though many idols were being worshipped, some idols of note include Hubal (placed in the middle of the Kabah), Manat (near the Red Sea), and Al-Lat (in the city of Taif near Mecca). During the conquest of Mecca later after the revelations of Muhammad, 360 idols were found around the Kabah; they were then removed and burned up.
Judaism: Jews migrated to Arabia in two major phases, once in 587 BC when the Jews left Palestine, and then again in 70 AD with the Roman occupation of Palestine. There were organized into various tribes and villages during the time of Muhammad. Some of the famous Jewish tribes included the Khabeer, Quraizah, and Qainuqa.
Christianity: Christianity entered Arabia via the Ethiopian and Roman colonists in the area. The Ethiopians managed to enter Yemen and even built a church with the aim of redirecting pilgrims from Mecca toward Yemen, but they were unsuccessful in their attempt. Followers of Christianity and Judaism were minor compared to the following of Arab Paganism.
Muhammad would later, at the age of 40, receive the revelation detailing the teachings of Islam. These teachings would change the face of Mecca forever.