Who says that? Only few plugins don't need to be adjusted, because they don't use the Joomla! API or only parts of the API which were not changed. Look into the JED and check how many extensions are marked as Joomla! 2.5 compatible and not Joomla! 3...
As long as the developer hasn't stated that the extension works in Joomla! 3, you shouldn't expect that it works!
You should wait until a Joomla! 3 version is released by the developer.
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