One of my S3 Antminers is not mining!
I mean it's mining at 0.5 GH/s or so!!!
Also the Fan speed of both blades is very huge!!!
Any clue what's wrong with it?

I would open the unit, check the connections, and inspect the boards.
If you don't find anything, I would re-flash the firmware.
You can also try turning the unit on and off, and reboots from the GUI.
I did exactly all you told!
Inspected everything and no problem found FYI the unit was mining for several days normally but start to act like that from last night.
I also changed the control unit with a new one but same thing happen.
Last thing that I did was removing one blade and now the unit is working half load with one blade.
So seems that one blade had problem.