Please try this : Connect the supplied OTG cable to the XP-1 and then switch on the device. Connect the other end of the OTG cable to the S3. Check if it works. If it doesn't work, disconnect the OTG cable from the S3 and restart (reboot) the phone. After the S3 has rebooted, connect the cable to the phone again and check if it works. I tried it this way and it seems to work. I suspect the supplied OTG cable is not the same as those commonly available OTG cables in the market. The reason is because when I tried to charge the phone with the XP-1, it will only work with the supplied OTG cable. Normal OTG cables does not seem to work.
Also, I noticed that once I switched on the phone charger function, the connection with the S3 is cut off. I need to restart my phone again in order to connect with the XP-1.