If you have tried the known method to get the 3gs on new bootrom 4.2.1 and ipad baseband out of revocery mode and you are still stuck in recovery mode look at this post to get it back up and going. Will work even if you are in a constant dfu black screen. Its longer but it will get it up for sure.

Solution to fix iphone 3g,3gs 4.2.1 with (new boot rom) recovery loop fix 1000% no bs