Are Flex 4.5 mobile compiled ipa's only compatible with iPhone 3gs and above? is there a way to get them working on a 3g phone?
I am not entirely sure, but I think it's 3GS and above that is supported by Flex.
Flex 4.5 doesn't officially support iOS. Make sure you are using Flex 4.5.1 which adds support for iOS and BlackBerry Tablet OS platforms.
Yes iPhone 3GS is the minimum requirement.