In this link you will find a page that has a timeline of events of the April 3rd Incident, as well as several pages that cover major events along the timeline with more detail and background information.

An excerpt from the section "Jeju’s political climate following liberation":

Although the People’s Committees in other regions were either dissolved by the USAMGIK or operated under different names, the Jeju People’s Committee remained intact and enjoyed strong support. [...] The People’s Committee in Jeju demonstrated leadership in the period following independence. [...] E. Grant Meade, a USAMGIK officer, said, “The Jeju People’s Committee was the only political party in the island and the only organization acting like a government.” [...] In every aspect, the Jeju People’s Committee was the only political party and the only government in Jeju.

A testimony of Kang Soon-hyun, a middle school teacher who was 27 at the time of the incident, from the section "Life on Jeju after liberation":

After liberation, the people who had left their hometowns as conscripts, forced laborers or workers returned with great expectations. But there was no way to earn money on Jeju. Although some people say that Jeju 4·3 occurred because of the South Korean Labor Party, that was a secondary reason. People resisted because, even after liberation, the pro-Japanese had not been punished, there were no jobs available and they were being suppressed by a corrupt police force.

The article continues:

Even in this severe situation, the corrupt officers who had worked for the Japanese kept their positions of authority under the USAMGIK and continued to exploit the Jeju people. Islanders, particularly the youth who felt the situation most keenly, were outraged and began to demand justice.

Eventually, in light of several factors (including right-wing paramilitary Northwest Youth League's vigilante attacks on Jeju islanders, as well as the division of Korea and controversial elections being held), acts of resistance occurred, described on the page "Day of infamy: April 3, 1948":

350 individuals on Jeju attacked police stations in part due to strong opposition to the division of the country. This is how, why and when the Jeju 4·3 Uprising and Massacre began. On this day, 12 of the 24 police stations on the island were attacked and 14 people were killed. The armed resistance announced: “Resist against Oppression!”

Skipping ahead to the final section, here is an example of the scorched-earth policy taken by the military government to suppress people's resistance on Jeju:

The severe crackdown started Oct. 27, 1948, when Song Yo-chan, who was appointed the 9th regimental commander, announced in a decree that all land beyond 5 km from the coast is “hostile territory” and any individual entering the region “will be killed unconditionally.” This decree seriously violated human rights and resulted in the illegal killing of Jeju people. [...] most villages situated in the mid-region of Mt. Hallasan were burned and all villagers unable to leave were brutally killed. No exceptions were made even for women, children or the elderly. These killings were illegal even during war, and this merciless carnage continued unchecked.

At the start of the conflict, around 500 members of the armed resistance were identified by the military and the police. However, some 30,000 people were killed in order to punish those 500 people, and in the process many evil and unimaginable acts were committed on the island.

That is all I will include in this post, but there is much more information and background included in the page in the link.