What do the three explorers do at the top of the mountain?
There are actually two incidents of explorers climbing to the top of the mountain. The first occurs in Chapter 1 when Ralph, Jack, and Simon are exploring the island. The purpose of this expedition is to find out if the land is actually an island. As they climb to the top of the mountain they see tracks made by some animal, foreshadowing the fear of the beastie that later creates chaos. Blocking their way to top is a huge rock, that they send rolling down the mountain. This incident is the first of the three rock rolling episodes in the novel, the third of which ends in Piggy's death. When the boys reach the top, they survey the island, and Ralph pronounces, "This belongs to us." They are impressed with the boat-shaped island and its coral reef, and Jack finds the place where they landed. At this point the island seems friendly, and the boys are all friends.
On the second expedition, the mission is to find the beast that Sam and Eric spotted. The explorers are initially Simon, Ralph, and Jack, and a few little ones, but Simon and the little ones go back to find Piggy, and Roger takes Simon's place. In this expedition, the boys also find a rock, but only Jack rolls this one down the mountain while Ralph seems more focused on their mission. We see that Ralph has matured, but Jack has not. This time when they reach the top of the mountain, the boys see what they think is the beast. In actuality this is the dead parachutist, but because it is dark, the boys run without investigating. This incident causes the total break-up of the group as Jack soon leaves Ralph's group and forms a tribe of his own. The island now has a much more ominous tone.
If you are talking about the beginning when Simon, Jack and Ralph travel to the top of the mountain to explore. They just go to the top and look down onto the island, their motif to go to the top was to find out if the place they crashed was and island and what the rest of the place looks like.
they discover the parachutist but they belive its an ape/beast