Shiny locks, as in explicit developer-induced limitations on what Pokemon can be found Shiny, do not exist in Generation III. Shiny locks began in Generation V, with Black and White locking Reshiram, Zekrom and all event Pokemon.
However, though there are no Shiny locks, there is still one Pokemon that cannot be legitmately obtained its its Shiny form in Generation III, which is Celebi, as it is an event-exclusive Pokemon that was never distributed in its Shiny form throughout the generation. However, the legendary Pokemon found within the actual games themselves, such as Groudon and Kyogre, obviously don't have the event Pokemon limitation. You're free to start resetting for them.
Example of Shiny Rayquaza from Emerald
List of Shiny locks (scroll to bottom)
List of unobtainable Shiny Pokemon
