It would help if you provide more details, if you do a search for 'com.android.systemui' you'll see that it's a fairly common error message.
What were you doing when that error message popped up? Is it while using a specific app or just randomly shows up? Has this been a long-standing issue or did this just start happening recently?
Just some really general suggestions:
-- Power your S3 off, remove the back cover, and remove the battery. Leave it out for a few minutes before putting the battery back in and restarting. (... and since the back cover is off re-seat the microSD card and SIM card if you have one or both. Not likely related to that error message but it can help bypass other issues.)
-- Another thing to try is to restart your S3 in its Recovery Mode (power it off, restart while simultaneously holding down the Volume Down button, the Home button, and the Power button) and clearing the system cache partition. The system cache resides in its own separate partition so this won't affect your personal data.
-- Try starting your S3 in its Safe Mode. When in Safe Mode it will be running Android as is, without pre-loading any third-party apps you've installed. If you don't see that error message pop up in while in Safe Mode that's an indicator the problem is probably tied to some app you've installed. Then the problem is figuring out which one. If there's an app that you recently installed about the same time that error message started occurring than work off of that (or if you look at your Play Store app's history to see if some app was recently updated about that same time).