What was the third place the family decided to reside in? Why did they wish to shift there?

Once safely ashore, the Robinsons urgently need to find shelter. So they hastily construct a makeshift tent out of some sailcloth they've brought with them from their ship. This is only a temporary home, however, as the family are going to need adequate protection from both the elements and the numerous wild animals that roam the island.
Their next dwelling is a large tree-house, a much firmer, more enduring structure that protects them from wild animals. As the Robinsons explore more of their new surroundings, they begin to construct additional dwellings across the island. This makes it less likely that all their hard work will be undone by a sudden natural disaster. Although the tree-house is a marked improvement on the makeshift tent, it still offers insufficient protection from the elements. Fortunately, the Robinsons find a cave, which acts as a warm, safe home, as well as a handy place for storing provisions.