What is Jack preoccupied with in Lord of the Flies chapter 3?

I wouldn't have said that Ralph has a 'new' preoccupation in chapter three: Huts on the Beach. I can only assume that you mean hut building but he makes it quite clear in his conversation with Jack that the building of huts was agreed upon by all of the boys during their meeting. He also states that while many of the boys did build the first hut, much less bothered to build the second one and only Simon and himself were left to try and build a third hut. The whole process appears to have gone on for some time, probably from a day or two after their arrival up until the time that the conversation took place. So the hut building wasn't new and it wasn't exclusively Ralph's idea. Ralph made it quite clear from the very start that his prime concern was to get rescued and while he was waiting for the rescuers to arrive, to have fun. I think that as a leader he also felt a sense of responsibility and was aware that the littluns need the security of shelters and, in the event of bad weather, they would all appreciate them. Ralph is remarkably consistent throughout the novel and his goals don't really change.