Roman Fish ( supersonicpro44) using rice as a tool to fix water damaged devices is nothing but a terrible myth. It does not work. It is not even a good desiccant. Using rice will do nothing but waste valuable time. It does not fix your device! You are wasting time and rice.
So now that we got this out of the way time to try to fix your Nintendo 3DS. First off take it out of the rice. Stop trying to turn it on or to charge it. That could cause further damage on the board. Then you want to go ahead a disassemble it by using these guides. Once disassembled you do want to go ahead and clean the motherboard and all the connectors to it with a high grade (90%+) isopropyl alcohol. Use this guide to see how to properly clean things. It was written for an Apple iPhone, but all the points are still pertinent to your 3DS as well. Once you got it properly cleaned and inspected for any missing or burned looking components, reassemble it with a new battery. this is not optional and you will have to replace the battery since it will fail after the water damage. Only after you've done all this turn your #DS back on an re-evalaute for any damage. Water damage is the toughest to diagnose and the toughest to repair since virtually any part can have failed.