Who went to make sure the island was really an island?

Ralph, Jack, and Simon take part in the expedition to make certain they're on an island.

"Listen, everybody. I've got to have time to think things out. I can't decide what to do straight off. If this isn't an island we might be rescued straight away. So we've got to decide if this is an island. Everybody must stay round here and wait and not go away. Three of us--if we take more we'd get all mixed, and lose each other--three of us will go on an expedition and find out. I'll go, and Jack, and, and . . ." He looked round the circle of eager faces. There was no lack of boys to choose from. "And Simon."