Consult a dentist  get a xray done and see whether the tooth is normal or is impacted , if impacted get it removed or if it is erupting normal then find the cause for swelling.

First get it diagnosed from the dentist and get an X-Ray done. If the third molar is impacted then it needs to be removed otherwise the swollen mouth can be because of number of other reasons.

As you mentioned above your 3rd molar is coming , its normal to having pain in that area .sometimes it migh cause swelling as well , The best treatment for 3rd molar get it extracted(removal) For timebeing do rineses with luke warm water with pinch of salt 3-4 times a day . For temporary pain relife you can take tab Ketorol dt 2 tablets in half glass of water . For permanant relive you have to visit your dentist.

Get an X-ray done for that tooth to check if it is coming in the proper position. If it is impacted you may require removal of that tooth.