Iphone 3gs in dfu mode, itunes says to put in dfu mode. Tried many times but no luck.

I plug in my iphone 3gs into itunes and put it into dfu mode. Then, itunes says to put it in dfu mode even though it already is. So I restart the iphone 3gs into dfu mode many times, but no luck. I switch to a different computer, but no luck. I tried using an older version of itunes but still no luck.