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Think of it like a huge empty lake, with a river running into it that is partially blocked off by three dams that is being fed by a huge body of water. Gohan hasn't broken through the limits of Super Saiyan 2 specifically, but has accessed that power through different means entirely. In order to reach Super Saiyan 3, you need to still break through the limits of Super Saiyan 2 through some means, in the same way you need to break through the limits of Super Saiyan in order to achieve 2.
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