List the three boys who continue the search for beast what beast do they see?

I assume that you are referring to the three boys who climbed to the top of the mountain in chapter 7: Shadows and Tall Trees. The three boys were Ralph, Jack and Roger. They saw exactly the same thing that Samneric had earlier seen, the dead body of an airman still attached to his parachute. Just like Samneric, in the fading light of evening, they mistook the body, which moved as the wind caught its parachute, for the beast.