If we are discussing "the importance of place," then we are really talking about a setting that has two different facets: the setting of time and the setting of place. Since you have asked about the latter we will discuss it at length in regard to the three books you mention.
In Treasure Island, even though the Admiral Benbow Inn, Bristol, and the Hispaniola deserve to be mentioned, the most important aspect of the setting is the actual place called Treasure Island. This island, an uninhabited pirate haven in the West Indies, is home to Captain Flint's famous treasure. The novel was written around the map (instead of vice versa). There are quite a few specifics about the setting such as Treasure Island is shaped "like a fat dragon standing up." The names of the different areas of Treasure Island make first-time readers wonder if it is really a ship. It isn't. A few prominent names are: Fore-mast, Main-mast, Spy-glass, Mizzen-mast, Haulbowline Head, Kidd's Anchorage, Skeleton Island, North Harbor & South Harbor. Perhaps located near Mexico and in the Carribean Sea, Jim fails to reveal the precise location because "there is still treasure not yet lifted." When Jim and companions finally arrive they find few savages and animals (as they expected) on a tropical paradise with exotic plants and strange land formations not characteristic of the area. The rest of the novel follows Jim on his adventures with pirates all around this island.
Swallows and Amazons is similar in that it is also set in an imaginary place. Further, it alludes to Treasure Island continually (such as with the uncle's nickname of "Captain Flint"). Although the children's adventures are imaginary, the actual novel is set in the Lake District which is basically the mountains of northern England where Wordsworth once wrote. And, obviously, this is a VERY different location than the West Indies! The Blacketts and the Walkers (deemed the Swallows and the Amazons) have lots of pretend pirate adventures here amid camping and fishing, etc.
Vastly different in setting than the other two, The Bunker Diary is set exactly where the title says it is: in an underground bunker (holding kidnapping victims). We are left completely in the dark as to where the bunker is. Further, that is unimportant. The characters originally unite and try to escape, or at least evade the kidnapper's games. "The man upstairs," as the kidnapper is called, finally asks them to resort to cannibalism. Eventually all of them die for one reason or another and the novel ends, in the bunker, in mid-sentence.
Thus, in conclusion, I would say that Treasure Island and Swallows and Amazons are two settings that could easily be compared (and the positive nature of the adventures there can be expressed) whereas the setting of The Bunker Diary is a completely different place unto itself (and the negative foreboding tone, almost resorting to nihilism, is almost incomparable to the former two).