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When I had my S2 I bought Swiftkey for the keyboard. Now I upgraded to the S3 I downloaded it again but it will not let me keep it as default. If I restart the phone it defaults back to the Samsung keyboard.
This is a known bug in Jelly Bean. Are you running Jelly Bean on your S3?
The work around that I have used is to change the default keyboard to the one I want to use and then in Settings - Applicatin Manager - All press on Samsung Keyboard and press Force Stop.
This should stop is running until you power cycle the phone.
Good workaround, though for me I never had the issue and SwiftKey remains my default keyboard.
Which, to me, points to something going wrong with the initial set-up of SwiftKey on Jelly Bean. Works for some but not others.