I have an S3 and looked into this quite extensively. I abandoned the idea as it seemed more work than it was worth, especially after finding out you can't use an external audio source for video recording. The app Gene linked is what you need for USB audio. Just be careful about the OTG cables. Some USB devices are going to need more power than the 5V your phone can supply, so there's a type of OTG cable that splits data and power, running off an external battery pack like we use for everything else around here.