At the end of chapter three, what does "Simon wanders off and finds beauty" mean?
There is a significant difference here between Simon's perception and the perception of the others. In Chapter 3, Ralph and Jack argue over the necessities. Ralph insists on working on the shelters and keeping the fire going to ensure chance of rescue. Jack stresses the need to hunt for meat. Although Ralph's focus is more important than Jack's (they have fruit, rescue is paramount), the important thing here is that a power struggle is beginning.
While this power struggle beings (which will eventually lead to a division and a violent overtaking by Jack), there is Simon who has no interest in the power struggle. Simon is interested in building the shelters, and of being rescued, but he is the only one that sees the beauty of the island. While the others are fighting and looking for conflict, Simon looks for beauty, something positive.
Here, Simon emerges (to the reader) as a the peaceful one, the one who sees things differently. It is almost as if Simon, being quite intuitive, can sense that this beginning power struggle will progress to something evil. So, he retreats to the jungle, to a place where he is away from that evil sense. He is literally and figuratively more in touch with the beauty of nature than he is with the boys' arguments. The first line implies the island's foliage embracing him:
The creepers and the bushes were so close that he left his sweat on them and they pulled together behind him. When he was secure in the middle he was in a little cabin screened off from the open space by a few leaves.
The descriptions of the natural splendor of the island are poetic and this illustrates Simon's perception of the island's beauty, which is in stark contrast to the frustrations and conflicts arising among the other boys, and particularly between Jack and Ralph.
This establishes Simon as an innocent, intuitive boy who can sense how things are progressing.