You can set your “base SDK” to the latest (iOS 4.2) and “iOS deployment targets” in iOS 3.0. Make sure you are not using code that is not available for iOS 3.0.
Personally, I think it would be fine to just set it to 3.1.3 ...
EDIT I haven't tried it yet, but it looks like this would be a useful question for loved ones. find unsupported apis with os version
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