The three boys are littleuns. Golding has just explained that they have a society all of their own. He tells the reader that...
"They obeyed the summons of the conch, partly because Ralph blew it, and he was big enough to be a link with the adult world of authority; and partly because they enjoyed the entertainment of the assemblies. But otherwise they seldom bothered with the biguns and their passionately emotional and corporate life was their own." (pg 59)
The littleuns had built this elaborate system of sand castles at the bar of the little river. It had complex marks,tracks, and even railway lines that allowed them to play for hours and absorbed their time while on the island. The three playing with the sand castles in chapter 4 are Henry, Percival, and Johnny. Henry is the biggest of the three boys and he is related to the boy with the birthmark on his face who has not been seen since the night of the great fire. He is very gullible. If they told him his relative got on an airplane and left, he would have believed them. The other two boys, Percival and Johnny, are the two smallest boys on the island. The three of them are being obedient and peaceful now because they are intent on their sand castles and on their playtime.