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Could you please tell me the memory card capacity that you are using.(For eg 2GB, 4GB, 8GB etc).
These cards are compatible with the camera.
Please check whether the card lock switch is in lock position. If it is in lock position, just unlock it.
Click here to see the image of the SDHC card lock switch.
Let me know the results please.
These are the two checks what we have to do in such an eventuality.
Also since you have already tried with three different cards, there is no possibility that all cards are faulty.
Unfortunately, in such a case the camera has to be suspected. The only option is to send the camera for repair to an authorized agent.
That's the suggestion available in the manual as well.